Gracefully handle non-existing infractions

[ChrisLovering]

This closes #1375.

Catches the response code error and handles by raising a bad argument error

[coveralls]

Coverage decreased (-0.04%) to 56.593% when pulling acc8bcf on ChrisLovering:deal-with-unknown-infraction-IDs into 3d014d9 on python-discord:master.

[Xithrius]

Tested, looks good to me.

[ChrisLovering]

Following discussions in #dev-contrib I moved this handling into error_handler.py. I copied a similar pattern to what was above, which should allow us to handle different types of inner errors from ConversionError in future as needed. Please let me know if this wasn't the expected implemention.

[Xithrius]

I'm pretty sure this is what was wanted in implementation, thanks for the quick change!

I will test a bit more and approve if everything is handled.

[ChrisLovering]

My only concern is that it sorta duplicates the check on line 78, but that's from CommandInvokeError. I guess the comment on 95 should be changed to remove ConversionError too, looking at it again.

[Xithrius]

I do agree that it does seem to be a little duplicated, but there's nothing really we can do about it to my knowledge since they're two different outer errors.

Yes, that comment should be updated.