oops display doesn't make it clear whether the exception stopped the process

[Martin Pool]

In OOPS-2116INBOUNDEMAIL2 from bug 878140 a traceback is shown in a way that looks a lot like the exception caused the process to abort.

However, in fact (it seems) the oops was generated because of a logger.warning call, and this was just the previous exception that was raised and caught.

improvements:
 * explain whether this error was actually unhandled, or if it was just the last handled error
 * if the oops was caused by a logger message, show that message, and the traceback leading up to the logger call