Only support negotiated simplates for errors

This is a step on the way to addressing #279. It makes error handling a lot simpler if we don't look for {$code,error}{.$ext,.$ext.spt,.spt}, where $ext is infinitely variable. Instead we look just for {$code,error}.spt. Eventually we may decide to simplify non-error simplates the same way: only .spt, no special-casing of .$ext.spt on the filesystem (#286). For now though the next step for #279 would be to update the Accept header of the internal redirect in
delegate_error_to_simplate to enforce the media type that had been negotiated to begin with.