Improve tracebacks on exceptions

Use failure.Failure to recover our failure, which will give us a useful
stacktrace, unlike the rethrown exception.

synapse/http/server.py

@@ -28,6 +28,7 @@
 )
 
 from twisted.internet import defer
+from twisted.python import failure
 from twisted.web import server, resource
 from twisted.web.server import NOT_DONE_YET
 from twisted.web.util import redirectTo
@@ -131,12 +132,17 @@ def wrapped_request_handler(self, request):
                             version_string=self.version_string,
                         )
                     except Exception:
-                        logger.exception(
-                            "Failed handle request %s.%s on %r: %r",
+                        # failure.Failure() fishes the original Failure out
+                        # of our stack, and thus gives us a sensible stack
+                        # trace.
+                        f = failure.Failure()
+                        logger.error(
+                            "Failed handle request %s.%s on %r: %r: %s",
                             request_handler.__module__,
                             request_handler.__name__,
                             self,
-                            request
+                            request,
+                            f.getTraceback().rstrip(),
                         )
                         respond_with_json(
                             request,