Fix string type error issue (#1128)

src/nni_manager/rest_server/restHandler.ts

@@ -203,7 +203,7 @@ class NNIRestHandler {
                 res.send();
             } catch (err) {
                 // setClusterMetata is a step of initialization, so any exception thrown is a fatal
-                this.handle_error(err, res, true);
+                this.handle_error(NNIError.FromError(err), res, true);
             }
         });
     }