Previously a GID with an invalid `model_name` portion would through an
unhandled `NameError` when being parsed by `GlobalIdType`.

The error was thrown when calling `model_class` on the GID:

    GlobalID.new("gid://gitlab/invalid_name/7").model_class
    # => NameError: wrong constant name invalid_name

Which was due to `.model_class` calling `constantize` on the `String`.

This change handles this situation as a `GraphQL::CoercionError`.