CgitbMiddleware doesn't handle close() properly

[Xavid]

The CgitbMiddleware tries to yield an error page even after catching a
GeneratorExit (which happens when the iterator is .close()d).
yielding after close is invalid behavior for an iterator and results
in a "RuntimeError: generator ignored GeneratorExit" exception.

It seems like the right fix would be to add something like

except GeneratorExit:
if not error_on_close:
app_iter.close()

before the "except:" line in the definition of catching_iter().

(I tried to open a ticket in Trac, but even after creating an account
it wouldn't let me create a new ticket. Let me know if there's a
better way to report this.)

Cheers,
~Xavid