Fix error handling on session restore

Fix a number of failures that meant the excellent error handling
we had for failing to restore a session didn't work.

1. .catch on the promise rather than try/catch: it's async
2. Explicit cancel method in SessionRestoreErrorDialog that invokes
   onFinished with `false` because even with the catch fixed, this
   was getting the event as its first arg which is truthy, so
   clicking cancel still deleted your data.
3. DialogButtons: Don't pass onCancel straight into the button event
   handler as this leaks the MouseEvent through as an argument.
   Nothing is using it and it exacerbates failures like this
   because there are surprise arguments.

Fixes https://github.com/vector-im/riot-web/issues/6616

src/Lifecycle.js

@@ -215,7 +215,6 @@ function _restoreFromLocalStorage() {
 
     if (accessToken && userId && hsUrl) {
         console.log(`Restoring session for ${userId}`);
-        try {
         return _doSetLoggedIn({
             userId: userId,
             deviceId: deviceId,
@@ -223,10 +222,9 @@ function _restoreFromLocalStorage() {
             homeserverUrl: hsUrl,
             identityServerUrl: isUrl,
             guest: isGuest,
-            }, false).then(() => true);
-        } catch (e) {
+        }, false).catch((e) => {
             return _handleRestoreFailure(e);
-        }
+        }).then(() => true);
     } else {
         console.log("No previous session found.");
         return Promise.resolve(false);

src/components/views/dialogs/SessionRestoreErrorDialog.js

@@ -41,10 +41,14 @@ export default React.createClass({
         Modal.createTrackedDialog('Session Restore Error', 'Send Bug Report Dialog', BugReportDialog, {});
     },
 
-    _continueClicked: function() {
+    _onContinueClick: function() {
         this.props.onFinished(true);
     },
 
+    _onCancelClick: function() {
+        this.props.onFinished(false);
+    },
+
     render: function() {
         const BaseDialog = sdk.getComponent('views.dialogs.BaseDialog');
         const DialogButtons = sdk.getComponent('views.elements.DialogButtons');
@@ -81,8 +85,8 @@ export default React.createClass({
                     { bugreport }
                 </div>
                 <DialogButtons primaryButton={_t("Continue anyway")}
-                    onPrimaryButtonClick={this._continueClicked} focus={shouldFocusContinueButton}
-                    onCancel={this.props.onFinished} />
+                    onPrimaryButtonClick={this._onContinueClick} focus={shouldFocusContinueButton}
+                    onCancel={this._onCancelClick} />
             </BaseDialog>
         );
     },

src/components/views/elements/DialogButtons.js

@@ -39,6 +39,10 @@ module.exports = React.createClass({
         focus: PropTypes.bool,
     },
 
+    _onCancelClick: function() {
+        this.props.onCancel();
+    },
+
     render: function() {
         let primaryButtonClassName = "mx_Dialog_primary";
         if (this.props.primaryButtonClass) {
@@ -53,7 +57,7 @@ module.exports = React.createClass({
                     { this.props.primaryButton }
                 </button>
                 { this.props.children }
-                <button onClick={this.props.onCancel}>
+                <button onClick={this._onCancelClick}>
                     { _t("Cancel") }
                 </button>
             </div>