narrow bare except in twisted errback to Exception

[HrachShah]

narrow bare except in twisted errback to Exception

The errback handler for Deferred-to-Future conversion wrapped
failure.raiseException() (the standard way to re-raise a twisted
Failure as its underlying exception) plus a defensive
'raise Exception("errback called without error")' (in case the
Failure wrapped no exception at all). The bare 'except:' caught
every BaseException including KeyboardInterrupt and SystemExit.

The handler's intent is to capture the exception info so it can be
stashed on the Future (so the awaiter sees the right error), so
narrowing to 'except Exception' is the right shape: a real
KeyboardInterrupt raised inside the twisted reactor (e.g. the
caller pressed Ctrl-C in a different thread, or twisted itself
propagated Ctrl-C from a signal handler) should now propagate out
to the tornado event loop instead of being silently converted to
a Future exception that the awaiter would observe as a generic
'errback called without error' message. A real SystemExit raised
inside twisted (e.g. via a signal handler that the user wired
themselves, or a twisted plugin that called sys.exit on a
shutdown) similarly propagates to the IOLoop's atexit hooks
instead of being hidden inside a Future result. MemoryError and
RecursionError are not caught by 'except Exception' either, which
is the right behavior — a Future is the wrong place to attempt to
record a process-level resource exhaustion.