Fix echo handling and show a barebones toast on error

The EchoTransaction was wrongly assuming that it knew better than the caller for when the success condition was met, so the echo marking has been left an exercise for the caller. In this case, we mark when we finally receive the sync with the updated rules.

We also have to cancel previous transactions otherwise if the user mashes buttons we could forever show the toast, and that would be bad.

src/components/views/toasts/NonUrgentEchoFailureToast.tsx

@@ -0,0 +1,30 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import React from "react";
+import { _t } from "../../../languageHandler";
+
+export default class NonUrgentEchoFailureToast extends React.PureComponent {
+    render() {
+        return (
+            <div className="mx_NonUrgentEchoFailureToast">
+                {_t("Your server isn't responding to some <a>requests</a>", {}, {
+                    'a': (sub) => <a>{sub}</a>
+                })}
+            </div>
+        )
+    }
+}