Add a generic error page element for startup errors

Used by Riot to communicate configuration problems.

src/components/structures/GenericErrorPage.js

@@ -0,0 +1,38 @@
+/*
+Copyright 2019 New Vector Ltd
+
+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 PropTypes from 'prop-types';
+import {_t} from "../../languageHandler";
+
+export default class GenericErrorPage extends React.PureComponent {
+    static propTypes = {
+        message: PropTypes.string.isRequired,
+    };
+
+    render() {
+        return <div className='mx_GenericErrorPage'>
+            <div className='mx_GenericErrorPage_box'>
+                <h1>{_t("Error loading Riot")}</h1>
+                <p>{this.props.message}</p>
+                <p>{_t(
+                    "If this is unexpected, please contact your system administrator " +
+                    "or technical support representative.",
+                )}</p>
+            </div>
+        </div>;
+    }
+}