Improve the `Gitlab::OAuth::User` error message
What does this MR do?
Fixes a logging issue submitted by a customer at https://gitlab.zendesk.com/agent/tickets/50728.
The error saving the user is logged to application.log. Previously,
the entry had no context and was unusable - Error saving user: [Email address already taken]
. Adding the auth hash UID and email
makes the error more helpful.
For such a small logging change, do we need a changelog entry?