Feature: update errors so that they support unit enums rather than just backed enums

[lindyhopchris]

Previously the error interfaces were type-hinting BackedEnum which meant you couldn't use a regular UnitEnum for error codes. This has now been fixed.

Also added some new features:

1. Added code() method to the ListOfErrors interface so that you can get a single error code from the list.
2. Error objects now allow the key to be an enum or string, previously only strings were supported.
3. Updates to the KeyedSetOfErrors to handle enum keys.