Catch swob responses that are raised.

This lets us get rid of some really repetitive exception conversion
code, like everybody that called common.utils.get_param() had to catch
a UnicodeDecodeError and turn that into returning HTTPBadRequest. Now
get_param() just raises HTTPBadRequest directly, and the __call__
methods in the account/container/object servers catch and return
it. All that "except UnicodeDecodeError" stuff goes away.

Refactored the path splitting and device validation in the object
server too.

There are other things that can benefit from this as well, but this
patch is big enough.

Change-Id: I2be96ef757d04bfd6af180cd9c92393c841db21f
Merge "Catch swob responses that are raised."