SL-19690: Follow up on Rye Mutt's fix for shutdown crashes.

Rather than continuing to propagate try/catch (Closed)
(aka LLThreadSafeQueueInterrupt) constructs through the code base, make
WorkQueueBase::post() return bool indicating success (i.e. ! isClosed()).
This obviates postIfOpen(), which no one was using anyway.

In effect, postIfOpen() is renamed post(), bypassing the exception when
isClosed().

Review existing try/catch blocks of that sort, changing to test for post()
returning false.