Use distinct exit codes for intentional exits

[pykereaper]

Currently, all the intentional application exits look like this:

exit("Please give an email address.")

This causes all exit codes to be 1. That's the same exit code when an Error is raised. This means, it is impossible to distinguish programmatically, if the application crashed, or halted gracefully. That can be important, when this application is called e. g. from within a script.

This pull request fixes this issue, by splitting all exit(<string>) occurrences in a print(<string>) call, followed by a exit() with a suitable code from the os package. For example:

print("Please give an email address.")
exit(os.EX_IOERR)

I would highly appreciate, if this pull request would be merged. I can't imagine a scenario, where this change has any negative effect.