Adds useful message to UserSettingsProxy.

This should help in debugging the associated bug as we don't know which
calling code is attempting to observe a setting for a user that was
deleted a long time ago and we need to make sure we stop that from
happening.

Bug: 362308685
Test: deployed on phone and verified that System UI seems to work
Flag: EXEMPT only adds extra message to stacktraces
Change-Id: Ifb9a92e7abd31c89f8fa6c44e67e40dc07ed9c0d