login: Show error dialog on get-server-settings failure

Fixes-partly: zulip#35

Author: chrisbobbe

lib/widgets/login.dart

@@ -146,7 +146,13 @@ class _AddAccountPageState extends State<AddAccountPage> {
     });
     try {
       serverSettings = await getServerSettings(realmUrl: url!);
-      // TODO(#35) report errors
+    } catch (e) {
+      if (!context.mounted) return;
+      // TODO(#35) give more helpful feedback; see `fetchServerSettings`
+      //   in zulip-mobile's src/message/fetchActions.js. Needs #37.
+      showErrorDialog(context: context,
+        title: 'Could not connect', message: 'Failed to connect to server: $url');
+      return;
     } finally {
       setState(() {
         _inProgress = false;