Feature request: "Unsupported value" error message - show supported values

[adamtheturtle]

Relevant code:

marshmallow-oneofschema/marshmallow_oneofschema/one_of_schema.py

Line 169 in a17d5c1

{self.type_field: ["Unsupported value: %s" % data_type]}

Rather than "Unsupported value: 'foo'", I'd like to show the supported values, similar to how Marshmallow's OneOf field type shows Must be one of: FOO.