Fix for issue #543: updated all exit calls with a unique return code

[danielritchie]

_deploy/lib/ml.rb_ presently exits with errors using "exit!" This forces an immediate exit that bypasses handling and returns "1". Using this method also causes something strange to happen with the output of the script, preventing redirection and other concerns See: Issue #543

All "exit!" calls have been replaced with "exit #" as I couldn't see any reason why the use of return codes would be problematic. I believe this maintains all existing functionality, and is a desirable enhancement.

There was a single "exit" call which was also replaced with a unique return code, "exit 10" - Note that this will change exiting behavior ("exit" alone will return 0). Although I expect this will be a desired addition, I wanted to call it out since it does change the return code.

More detail from Ruby docs:
exit vs. exit!

[danielritchie]

With unique return codes being available, it seemed worthwhile to pass them back to the parent script.

Previous behavior:

# ./ml bootstrap
ERROR MESSAGE
# echo $?
1

With this change:

# ./ml bootstrap
ERROR MESSAGE
# echo $?
7

[danielritchie]

Closing - Realized I need to submit an updated pull request to the dev branch

[danielritchie]

Updated pull request: #546