Unify CLI exit codes

[joshmoore]

The exit codes used throughout the CLI plugins are assigned randomly. Though it would be a breaking change, it would be useful to define a much smaller number of codes that users can expect.

see:

• https://forum.image.sc/t/omero-cli-exit-status/44110
• https://github.com/ome/openmicroscopy/pull/6003 (original work on capturing codes)
• https://github.com/ome/omero-py/pull/266 (more captured codes)

options:

• HTTP-like status codes
• https://docs.python.org/3/library/errno.html
• https://pypi.org/project/exitstatus/

Additionally as seen in https://github.com/ome/openmicroscopy/pull/6003#issuecomment-713445137 values should be reduced to less than 256.

[manics]

A starting point (and non-breaking change) could be a top-level enum that defines ALL allowed error codes in omero-py? i.e. an omero version of the Python errno module. <Edit actually ready your description properly now and you've already said this :-) >

[joshmoore]

I'd think that'd be a good addition, but I imagine plugins will need the ability to add. (i.e. not an enum but a registry)