Don't blow on bundle update gem when lock is not present.

* rename `not_found_message` to `gem_not_found_message`
* add `lock_not_found_message`
* add `GemfileLockNotFound` error with 22 status code

# Before

```
[retro@localhost  testgem (master #)]❤ bundle update ryba
Unfortunately, a fatal error has occurred. Please see the Bundler troubleshooting documentation at http://bit.ly/bundler-issues. Thanks!
/home/retro/.rvm/gems/ruby-2.0.0-p353/gems/bundler-1.6.0.pre.1/lib/bundler/cli.rb:316:in `update': undefined method `specs' for nil:NilClass (NoMethodError)
        from /home/retro/.rvm/gems/ruby-2.0.0-p353/gems/bundler-1.6.0.pre.1/lib/bundler/vendor/thor/command.rb:27:in `run'
        from /home/retro/.rvm/gems/ruby-2.0.0-p353/gems/bundler-1.6.0.pre.1/lib/bundler/vendor/thor/invocation.rb:121:in `invoke_command'
        from /home/retro/.rvm/gems/ruby-2.0.0-p353/gems/bundler-1.6.0.pre.1/lib/bundler/vendor/thor.rb:363:in `dispatch'
        from /home/retro/.rvm/gems/ruby-2.0.0-p353/gems/bundler-1.6.0.pre.1/lib/bundler/vendor/thor/base.rb:440:in `start'
        from /home/retro/.rvm/gems/ruby-2.0.0-p353/gems/bundler-1.6.0.pre.1/lib/bundler/cli.rb:9:in `start'
        from /home/retro/.rvm/gems/ruby-2.0.0-p353/gems/bundler-1.6.0.pre.1/bin/bundle:20:in `block in <top (required)>'
        from /home/retro/.rvm/gems/ruby-2.0.0-p353/gems/bundler-1.6.0.pre.1/lib/bundler/friendly_errors.rb:5:in `with_friendly_errors'
        from /home/retro/.rvm/gems/ruby-2.0.0-p353/gems/bundler-1.6.0.pre.1/bin/bundle:20:in `<top (required)>'
        from /home/retro/bin/bundle:23:in `load'
        from /home/retro/bin/bundle:23:in `<main>'
[retro@localhost  testgem (master #)]❤ echo $?
1
```

# After

```
[retro@localhost  testgem (master #)]❤ dbundle update ryba
This Bundle hasn't been installed yet. Run `bundle install` to update and install the bundled gems.
[retro@localhost  testgem (master #)]❤ echo $?
22
```