Added EnumGeneratorException

To be used if any of the expectations of the model parser aren't met. Currently just uses .Single calls.
Remove all use of .Single, check data and throw instead

Having problems with this block, so hopefully this will
provide better troubleshooting information.
Currently unable to reproduce the bug I saw but I know it's
in here somewhere!
Wrap troublesome name finder in try-catch to add context
Bump to v1.2.1 + release notes.