Problem with the ExitCode enum

[giovannipizzi]

The changes introduced in #1648 are partially incorrect.

For instance, calling sys.exit(ExitCode.UNKNOWN)

1. prints ExitCode.UNKNOWN on the screen before exiting (this is because ExitCode.UNKNOWN is a special type and not simply its value. One e.g. should do sys.exit(ExitCode.UNKNOWN.value). It would be ideal if we find a way to have an enum passing directly its values rather than going in the code and putting a .value everywhere, but I don't know if it's possible
2. always exits with error code 1 as sys.exit() does not know how to get the value from that object

The same problem in e.g. echo_critical (that I would still like to be called echo_critical_exit ;-) ), even if point 2 is less apparent because ExitCode.CRITICAL is 1

[sphuber]

Fixed in PR #1654