Simplify rendering and error handling

It's probably easiest to view this [without whitespace changes](https://github.com/olark/txroutes/pull/6/files?w=1).

This simplifies the weird recursion and flags.  It also removes the "is it an Exception or a Failure" logic in `_render_error` by deprecating that method (maybe we should just remove it?) and replacing it with a `_render_failure` method that will only receive a Failure instance.

This is based off of the logging in #5, so that PR should be merged that first.