User: better error message when getActorId fails.

This changes User::getActorId() to include the user name and id
when throwing an exception. This doesn't solve the problem
reported in T211450, but should allow the the probelmatic user
name to be identified.

Bug: T211450
Change-Id: Ie83ce6ad6b5ef18ea44a52e204f580cd9c992148
(cherry picked from commit db3e7f8b7e81eed410e6f82f939038768efd27ce)