Allow extensions to indicate a username doesn't exist

Provide a way for extensions to nicely handle when a username doesn't
exist, during the login process. This only is obviously only for the
case when we know why it doesn't exist (it was renamed, deleted, etc.)

See I06b9b6322e408868f516aeabd61c6580f304e009 for CentralAuth use case.

Bug: 67995
Change-Id: If48d59afa63ace68c147eca952f1d4f43acc105f
Merge "Allow extensions to indicate a username doesn't exist"