the former, remember it and raise it after calling _cleanup.  If
abort_sub() or tpc_abort() in _cleanup() also raise exceptions, log
them but don't propagate them.

This stops stray output produced by tests testExceptionInTpcAbort and
testExceptionInSubAbortSub.

Grrrrr:  I haven't yet been able to figure out how to get logging to work
in ZODB, so haven't yet been able to check that the new logging is
reasonable.