Add verbose backend status helper

This allows the same verbose information to be logged in the cases where
it is of use. Mostly this information can be used to figure out why a
certain session was closed.

server/modules/routing/readwritesplit/rwsplitsession.cc

@@ -1182,6 +1182,7 @@ bool RWSplitSession::handle_error_new_connection(DCB* backend_dcb, GWBUF* errmsg
         MXS_ERROR("Unable to continue session as all connections have failed and "
                   "new connections cannot be created. Last server to fail was '%s'.",
                   backend->name());
+        MXS_INFO("Connection status: %s", get_verbose_status().c_str());
     }
 
     return ok;