raise instead of falling through on unknown desc arg

[nburns]

Raising instead of returning the original value is much safer and gives users the ability to know when the arg passed to desc is not handled, instead of silently ignoring the intent.

Fix for: https://github.com/ponyorm/pony/issues/646

[nburns]

I wasn't sure how to run the tests, pytest after clone kinda worked but some instructions/help on contributing would be great.

[nburns]

closing this because of no response, the issue is still valid