RFA: add python exception subclasses

[Tom Tromey]

>>>>> "Tom" == Tom Tromey <tromey@redhat.com> writes:

Tom> This patch implements this idea.  It adds some new subclasses of
Tom> RuntimeError, including one specifically for MEMORY_ERROR.  Then it
Tom> arranges for the memory error exception to be treated specially by the
Tom> pretty-printing code.

I am checking this in now.

I don't think it fully fixes the problem, but it is a good step.

I think the other part is fixing the existing printers so that their
iterators more gracefully handle a MemoryError thrown when computing a
value.

Tom