Downgrade ResourceOAuth2TokenMixin error to warning

Review Request #9124 — Created Aug. 11, 2017 and submitted — Latest diff uploaded

Information

Djblets
release-0.10.x
5433a4f...

Reviewers

The OAuth2 scopes code logs an error if a resource does not inherit fom
the OAuth2 token mixin. We are downgrading this to a warning to seem
less scary (and because its not actually an error).

Confirmed the error no longer appears and a warning appears instead.