Add verbose logging for session command failures

If the routing of a session command fails due to problems with the backend
connections, a more verbose error message is logged. The added status
information in the Backend class makes tracking the original cause of the
problem a lot easier due to knowing where, when and why the connection was
closed.

server/modules/routing/readwritesplit/rwsplit_session_cmd.cc

@@ -78,6 +78,7 @@ static void discard_if_response_differs(SRWBackend backend,
                     STRPACKETTYPE(cmd),
                     query.empty() ? "<no query>" : query.c_str());
         backend->close(mxs::Backend::CLOSE_FATAL);
+        backend->set_close_reason("Invalid response to: " + query);
     }
 }