sidekiqctl does not check for PIDfile existence. Capistrano deployment fails as well.

I am deploying sidekiq with Capistrano 3 and got weird errors in case where worker process was dead and PID file did not exist. For some reason, the exception was raised despite the fact that PIDfile existence is checked on initialisation of Sidekiqctl.

Example of such situation:
```
 INFO [a826c429] Running /usr/local/rvm/bin/rvm 2.0.0-p247@ntpscanner do bundle exec sidekiqctl stop /home/deployer/ntp_scanner/current/tmp/pids/sidekiq.pid 10 on XXXXXXXXXX.COM
DEBUG [a826c429] Command: cd /home/deployer/ntp_scanner/current && /usr/local/rvm/bin/rvm 2.0.0-p247@ntpscanner do bundle exec sidekiqctl stop /home/deployer/ntp_scanner/current/tmp/pids/sidekiq.pid 10
DEBUG [a826c429]  /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/gems/sidekiq-2.17.5/bin/sidekiqctl:43:in `read': No such file or directory - /home/deployer/ntp_scanner/current/tmp/pids/sidekiq.pid (Errno::ENOENT)
DEBUG [a826c429]    from /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/gems/sidekiq-2.17.5/bin/sidekiqctl:43:in `pid'
DEBUG [a826c429]    from /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/gems/sidekiq-2.17.5/bin/sidekiqctl:54:in `block in stop'
DEBUG [a826c429]    from /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/gems/sidekiq-2.17.5/bin/sidekiqctl:52:in `times'
DEBUG [a826c429]    from /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/gems/sidekiq-2.17.5/bin/sidekiqctl:52:in `stop'
DEBUG [a826c429]    from /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/gems/sidekiq-2.17.5/bin/sidekiqctl:21:in `initialize'
DEBUG [a826c429]    from /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/gems/sidekiq-2.17.5/bin/sidekiqctl:78:in `new'
DEBUG [a826c429]    from /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/gems/sidekiq-2.17.5/bin/sidekiqctl:78:in `<top (required)>'
DEBUG [a826c429]    from /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/bin/sidekiqctl:23:in `load'
DEBUG [a826c429]    from /home/deployer/ntp_scanner/shared/bundle/ruby/2.0.0/bin/sidekiqctl:23:in `<main>'
cap aborted!
bundle stdout: Nothing written
bundle stderr: Nothing written
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/command.rb:94:in `exit_status='
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/backends/netssh.rb:142:in `block (4 levels) in _execute'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/channel.rb:551:in `call'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/channel.rb:551:in `do_request'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:561:in `channel_request'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:465:in `dispatch_incoming_packets'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:221:in `preprocess'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:205:in `process'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:169:in `block in loop'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:169:in `loop'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:169:in `loop'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/channel.rb:269:in `wait'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/backends/netssh.rb:164:in `block (2 levels) in _execute'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/channel.rb:514:in `call'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/channel.rb:514:in `do_open_confirmation'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:545:in `channel_open_confirmation'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:465:in `dispatch_incoming_packets'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:221:in `preprocess'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:205:in `process'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:169:in `block in loop'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:169:in `loop'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/net-ssh-2.8.0/lib/net/ssh/connection/session.rb:169:in `loop'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/backends/netssh.rb:166:in `block in _execute'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/backends/netssh.rb:123:in `tap'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/backends/netssh.rb:123:in `_execute'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/backends/netssh.rb:66:in `execute'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sidekiq-2.17.5/lib/sidekiq/tasks/sidekiq.rake:80:in `block (5 levels) in <top (required)>'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/backends/abstract.rb:81:in `within'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sidekiq-2.17.5/lib/sidekiq/tasks/sidekiq.rake:76:in `block (4 levels) in <top (required)>'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sidekiq-2.17.5/lib/sidekiq/tasks/sidekiq.rake:35:in `block in for_each_process'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sidekiq-2.17.5/lib/sidekiq/tasks/sidekiq.rake:34:in `times'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sidekiq-2.17.5/lib/sidekiq/tasks/sidekiq.rake:34:in `for_each_process'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sidekiq-2.17.5/lib/sidekiq/tasks/sidekiq.rake:74:in `block (3 levels) in <top (required)>'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/backends/netssh.rb:54:in `instance_exec'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/backends/netssh.rb:54:in `run'
/Users/marek.skrobacki/.rvm/gems/ruby-2.1.0@rscan/gems/sshkit-1.3.0/lib/sshkit/runners/parallel.rb:12:in `block (2 levels) in execute'
Tasks: TOP => sidekiq:stop
(See full trace by running task with --trace)
 $
```

While ugly stack trace instead of an error is usually not a huge problem, I felt it should be fixed hence this PR.


Second, bigger problem is that sidekiqctl returns non-zero exit status if PID does not exist and that stops code deployment if using built-in Capistrano integration tasks. I addressed it with a changes in `lib/sidekiq/tasks/sidekiq.rake`.

Following this change, the deploy task behaves as expected - it produces message about non-existent PID file and continues the deployment process:
```
INFO [6b656737] Running /usr/local/rvm/bin/rvm 2.0.0-p247@ntpscanner do bundle exec sidekiqctl stop /home/deployer/ntp_scanner/current/tmp/pids/sidekiq.pid 10 on my.target.host.com
DEBUG [6b656737] Command: cd /home/deployer/ntp_scanner/current && /usr/local/rvm/bin/rvm 2.0.0-p247@ntpscanner do bundle exec sidekiqctl stop /home/deployer/ntp_scanner/current/tmp/pids/sidekiq.pid 10
DEBUG [6b656737]  Pidfile /home/deployer/ntp_scanner/current/tmp/pids/sidekiq.pid does not exist
 INFO [6b656737] Finished in 2.745 seconds with exit status 1 (failed).
DEBUG [012509f9] Running /usr/bin/env if test ! -d /home/deployer/ntp_scanner/current; then echo "Directory does not exist '/home/deployer/ntp_scanner/current'" 1>&2; false; fi on my.target.host.com
DEBUG [012509f9] Command: if test ! -d /home/deployer/ntp_scanner/current; then echo "Directory does not exist '/home/deployer/ntp_scanner/current'" 1>&2; false; fi
DEBUG [012509f9] Finished in 0.229 seconds with exit status 0 (successful).
 INFO [e8f0c208] Running /usr/local/rvm/bin/rvm 2.0.0-p247@ntpscanner do bundle exec sidekiq -d -i 0 -P /home/deployer/ntp_scanner/current/tmp/pids/sidekiq.pid -r /home/deployer/ntp_scanner/current/ntp_distributed_scanner.rb -L /home/deployer/ntp_scanner/current/log/sidekiq.log on my.target.host.com
DEBUG [e8f0c208] Command: cd /home/deployer/ntp_scanner/current && /usr/local/rvm/bin/rvm 2.0.0-p247@ntpscanner do bundle exec sidekiq -d -i 0 -P /home/deployer/ntp_scanner/current/tmp/pids/sidekiq.pid -r /home/deployer/ntp_scanner/current/ntp_distributed_scanner.rb -L /home/deployer/ntp_scanner/current/log/sidekiq.log
```