add an asynchronous exception class

[Paul Rubin]

"Fredrik Lundh" <fredrik at pythonware.com> writes:
> PEP 348 addresses this by moving special exceptions out of the
> Exception hierarchy:
> 
>     http://www.python.org/peps/pep-0348.html

I see that suggestion was rejected (it needed changing the semantics
of "except:").  Also, PEP 348 was rejected and is a huge, complex
reorganization of the whole exception system.  This is cited:

  http://mail.python.org/pipermail/python-dev/2005-August/055423.html

but it talks about distinguishing terminating from non-terminating
exceptions, whatever that means.  (Won't any uncaught exception like
ValueError terminate the program)?

I realize now that exceptions arising from signals are also asynchronous
(http://docs.python.org/lib/module-signal.html).  So that's another
place where we'd see user-defined asynchronous exceptions: signal
handlers should raise them instead of raising ordinary exceptions.