Add test client helpers for catching Response

If you set raise_immediately=False when testing, then who knows what
you're going to get? When a test raises an exception you want exactly
that exception, not a downstream wrapped who knows what. So instead of
recommending overriding raise_immediately per-test, here are a couple
extra helpers: GxT and PxST, which are just like GET and POST but they
expect to have an exception raised, and they return that.
Add a load_resource helper to test client

This is coming upstream from Aspen, and required factoring out a
get_resource_for_request function from the get_response_for_resource
algorithm function.
Fix regression with nice error messages

At some point I had accidentally changed to looking for exc_info.html
instead of error.html.
Here's a failing test for an error handling bug

The attempts to fix up error handling by scabbing on the error code
were, shall we say, misguided.
Fix a regression in error handling

We weren't dealing properly with the response object when delegating
error handling to a simplate. We were trying to patch it up ad hoc but
it needed to be passed into resource.respond (and resource didn't need
to be on request).