Add path into extras

[gdassori]

Invalidating extras doesn't provide the involved property name into ValidationError.path, as other validators seems do.

[Julian]

Hey! Thanks so much for the PR.

The path attribute is supposed to be the path into the instance where the error occurred. That isn't the case here, you're passing a set of property names to it (not a tuple, by the way, in case that was what you meant).

There's an issue that's been open to add support for an extra_information field on ValidationErrors in #119 which you might be interested in, that would cover this case. I'm going to close this as a duplicate but feel free to follow up over there if you're interested. Thanks again!