Bug 755382: Switch to contextless error handlers.

DO NOT MERGE

This is simply a proof of concept of switching our error-handling views to supply no template context. Spoiler: it breaks a lot of things, because we have error templates which rely on having a bunch of context available to them.

Questions to answer from this:

1. Is it worth the effort of cleaning up our error templates, to get the additional safety of contextless errors?
2. Should all of our error views be contextless, or just a subset? (500 should absolutely be contextless, 404 probably should be, 403 is debatable).