Previously, we relied on raising a swob.HTTPException from within a
wrapped wsgi.input and just hoping that it either
- comes uncaught all the way back through the pipeline, or at least
- gets translated back to a 422 response.
This was brittle, however; if any middleware between s3api and the
proxy took s3api's approach of turning unexpected responses into 500s,
for example, it all breaks down.
Take a cue from eventlet's Timeout and create a new s3api-specific
BaseException (which does *not* inherit from Exception) to improve
our ability to cut through all the layers.
Change-Id: I9924ff3b8d7d246631fe61b916823e028e2c01f2
f5e422eb78