Fix PeerConnector handling of last-resort callback requirements

Fix PeerConnector handling of last-resort callback requirements

... and simplify the corresponding code.

Events like handleStopRequest() and callException() stop the job but
should not be reported as a BUG (e.g., it would be up to the
callException() to decide how to report the caught exception). There
might be (or there will) be other, similar cases where the job is
stopped prematurely for some non-BUG reason beyond swanSong() knowledge.

The existence of non-bug cases does not mean there could be no bugs
worth reporting here, but until they can be identified more reliably
than all these benign/irrelevant cases, reporting no BUGs is a (much)
lesser evil.