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?

See merge request !7851