Don't force-logout the user if reading localstorage fails

Give them a modal dialog to give them a chance to abort.

src/components/views/dialogs/SessionRestoreErrorDialog.js

@@ -0,0 +1,74 @@
+/*
+Copyright 2017 Vector Creations 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 sdk from '../../../index';
+import SdkConfig from '../../../SdkConfig';
+import Modal from '../../../Modal';
+
+
+export default React.createClass({
+    displayName: 'SessionRestoreErrorDialog',
+
+    propTypes: {
+        error: React.PropTypes.string.isRequired,
+        onFinished: React.PropTypes.func.isRequired,
+    },
+
+    _sendBugReport: function() {
+        const BugReportDialog = sdk.getComponent("dialogs.BugReportDialog");
+        Modal.createDialog(BugReportDialog, {});
+    },
+
+    _continueClicked: function() {
+        this.props.onFinished(true);
+    },
+
+    render: function() {
+        const BaseDialog = sdk.getComponent('views.dialogs.BaseDialog');
+        let bugreport;
+
+        if (SdkConfig.get().bug_report_endpoint_url) {
+            bugreport = (
+                <p>Otherwise, <a onClick={this._sendBugReport} href='#'>
+                   click here</a> to send a bug report.
+                </p>
+            );
+        }
+
+        return (
+            <BaseDialog className="mx_ErrorDialog" onFinished={this.props.onFinished}
+                    title='Unable to restore session'>
+                <div className="mx_Dialog_content">
+                    <p>We encountered an error trying to restore your previous session. If
+                    you continue, you will need to log in again, and encrypted chat
+                    history will be unreadable.</p>
+
+                    <p>If you have previously used a more recent version of Riot, your session
+                    may be incompatible with this version. Close this window and return
+                    to the more recent version.</p>
+
+                    {bugreport}
+                </div>
+                <div className="mx_Dialog_buttons">
+                    <button className="mx_Dialog_primary" onClick={this._continueClicked}>
+                        Continue anyway
+                    </button>
+                </div>
+            </BaseDialog>
+        );
+    },
+});