When an extension for AvatarServices doesn't include an implementation for get_avatar_urls_uncached or get_etag_data, it throws a 500 error. With the changes, we now return a default etag_data and return an empty string for the uncached avatar urls in addition to logging an error.

ran make html and the formatting changes appear.
tests for AvatarServices passed. Added 2 unit tests to ensure get_avatar_urls_uncached logs an error and get_etag_data returns the default [avatar.service_id, user.pk]