Log a stack trace from error responses

We're no longer seeing much in the way of 500 errors, but we're still getting some errors. 

This changeset logs a stack trace when you create an error response, which gives us some visibility of what's happening when the user sees an error.

I suspect we'll start wanting to turn this off for individual errors once it's been live for a little bit, but at least initially I think it's worth having for every request.