Bad URL causes ratelimit middleware to send traceback to the client

Two things:

1) we need a top level middleware that will catch log and return 500 on unhandled exceptions in middleware

2) we need to hunt down calls to split_path that don't have a try/except and make sure they're returning 404's

clayg@saio:~/swift$ curl http://localhost:8080//
Traceback (most recent call last):
  File "/usr/local/lib/python2.6/dist-packages/eventlet-0.9.9-py2.6.egg/eventlet/wsgi.py", line 335, in handle_one_response
    result = self.application(self.environ, start_response)
  File "/home/clayg/swift/swift/common/middleware/healthcheck.py", line 38, in __call__
    return self.app(env, start_response)
  File "/home/clayg/swift/swift/common/middleware/memcache.py", line 31, in __call__
    return self.app(env, start_response)
  File "/home/clayg/swift/swift/common/middleware/auth.py", line 86, in __call__
    1, 2, True)
  File "/home/clayg/swift/swift/common/utils.py", line 223, in split_path
    raise ValueError('Invalid path: %s' % quote(path))
ValueError: Invalid path: //

^ auth middleware line 86

clayg: your 1) is covered by https://code.launchpad.net/~notmyname/swift/except_middleware/+merge/39639