MXS-2938: Log session command info on routing failure

If the session command could not be routed, the log message should contain
the actual command that was routed. This makes failure analysis much
easier.

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -620,7 +620,8 @@ bool RWSplitSession::route_session_write(GWBUF* querybuf, uint8_t command, uint3
     }
     else
     {
-        MXS_ERROR("Could not route session command: %s. Connection status: %s",
+        MXS_ERROR("Could not route session command `%s`: %s. Connection status: %s",
+                  sescmd->to_string().c_str(),
                   attempted_write ? "Write to all backends failed" : "All connections have failed",
                   get_verbose_status().c_str());
     }