(bug 70497) Don't throw exception; show nicer error message for insufficient permissions

loadTopicTitle already did ->addError, which should expose the error
message in a nicer fashion. There were some other things to deal with
though:
* flow_errors needed to be included in template that would display it
* some later code paths still threw exceptions instead of quitting silently

Bug: 70497
Change-Id: I451bd22197462b0ae1d6865414694f35945df76a
Merge "(bug 70497) Don't throw exception; show nicer error message for insufficient permissions"