Cleanup top level request exception logging

Firstly, we always logged that the request was being handled via
`JsonResource._async_render`, so we change that to use the servlet name
we add to the request.

Secondly, we pass the exception information to the logger rather than
formatting it manually. This makes it consistent with other exception
logging, allwoing logging hooks and formatters to access the exception
information.

synapse/http/server.py

@@ -106,10 +106,10 @@ def wrap_json_request_handler(h):
             # trace.
             f = failure.Failure()
             logger.error(
-                "Failed handle request via %r: %r: %s",
-                h,
+                "Failed handle request via %r: %r",
+                request.request_metrics.name,
                 request,
-                f.getTraceback().rstrip(),
+                exc_info=(f.type, f.value, f.getTracebackObject()),
             )
             # Only respond with an error response if we haven't already started
             # writing, otherwise lets just kill the connection