Log command exit status

Some Capistrano users were confused about such messages:

    DEBUG [247d7a19] Command: [ -f /projects/myproject/releases/20131017094531/config/database.yml ]
    DEBUG [247d7a19] Finished in 0.114 seconds command failed.

And asked me if it's fine when 'command failed'

With this change users will see the exit status of command.