Refactor how we pick listings' content type.

There were a few different places where we had some repeated code to
figure out what format an account or container listing response should
be in (text, JSON, or XML). Now that's been pulled into a single
function.

As part of this, you can now raise HTTPException subclasses in proxy
controllers instead of laboriously plumbing error responses all the
way back up to swift.proxy.server.Application.handle_request(). This
lets us avoid certain ugly patterns, like the one where a method
returns a tuple of (x, y, z, error) and the caller has to see if it
got (value, value, value, None) or (None, None, None, errorvalue). Now
we can just raise the error.

Change-Id: I316873df289160d526487ad116f6fbb9a575e3de
Merge "Refactor how we pick listings' content type."