Log SystemExit in wsgi middleware when code != 0

[mattrobenolt]

SystemExit does not subclass Exception, so we don't catch this.
SystemExit manifests itself (at least) from gunicorn when gunicorn
attempts to gracefully shut down a worker process. See GH-675 for more
background.

Fixes: GH-675

[dcramer]

This might cause some duplicate events, but otherwise seems fine

[mattrobenolt]

In what cases do you think it'd cause duplicate events?

[dcramer]

If this propagates up to the system hook

[mattrobenolt]

Ah, lemme test that out and see what actually happens there. Not sure if the system hook ignores a SystemExit or not. My hunch is "yes", considering these errors weren't getting captured before.

[mattrobenolt]

import sys

def hook(*exc_info):
    print 'going down'

sys.excepthook = hook

raise SystemExit(1)

excepthook ignores SystemExit

[mitsuhiko]

Merged and also log KeyboardInterrupt which is currently also not handled. This serves a similar purpose for other servers.