add better OAuth error

Fixes #88

lib/controllers/client.dart

@@ -15,6 +15,7 @@ import "package:nexus/models/event.dart";
 import "package:nexus/models/event_context.dart";
 import "package:nexus/models/mentions_response.dart";
 import "package:nexus/models/gomuks_config.dart";
+import "package:nexus/models/gomuks_exception.dart";
 import "package:nexus/models/oauth_auth_code_response.dart";
 import "package:nexus/models/open_graph_data.dart";
 import "package:nexus/models/paginate.dart";
@@ -112,7 +113,7 @@ class ClientController extends AsyncNotifier<int> {
       final json = response.buf.toJsonAndFree();
 
       if (response.command.cast<Utf8>().toDartString() == "error") {
-        throw json;
+        throw GomuksException(json.toString());
       }
 
       return json;

lib/models/gomuks_exception.dart

@@ -0,0 +1,11 @@
+class GomuksException implements Exception {
+  final String message;
+  const GomuksException(this.message);
+
+  /// The Matrix error code, if gomuks returned an error in the form `M_CODE (HTTP 404): message`
+  String? get errorCode =>
+      RegExp(r"^(M_[A-Z_]+)").firstMatch(message)?.group(1);
+
+  @override
+  String toString() => message;
+}

lib/widgets/pages/select_server.dart

@@ -8,7 +8,9 @@ import "package:nexus/controllers/client.dart";
 import "package:nexus/controllers/client_id.dart";
 import "package:nexus/helpers/launch_helper.dart";
 import "package:nexus/main.dart";
+import "package:nexus/models/gomuks_exception.dart";
 import "package:nexus/models/homeserver.dart";
+import "package:nexus/models/oauth_auth_code_response.dart";
 import "package:nexus/widgets/pages/settings.dart";
 import "package:nexus/widgets/appbar.dart";
 import "package:nexus/widgets/divider_text.dart";
@@ -48,9 +50,26 @@ class const SelectServerPage({super.key}) extends HookConsumerWidget {
 
         if (context.mounted) {
           Future<void> tryLogin(Uri url) async {
-            final codeResponse = await ref.watch(
+            final OAuthAuthCodeResponse codeResponse;
+            try {
+              codeResponse = await ref.watch(
                 AuthUrlController.provider(url).future,
               );
+            } on GomuksException catch (error) {
+              if (error.errorCode != "M_UNRECOGNIZED") rethrow;
+              if (context.mounted) {
+                ScaffoldMessenger.of(context).showSnackBar(
+                  SnackBar(
+                    content: Text(
+                      "${url.host} doesn't support OAuth login, which Nexus requires. Try a different homeserver, or ask your homeserver's admin to enable OAuth.",
+                      style: .new(color: theme.colorScheme.onErrorContainer),
+                    ),
+                    backgroundColor: theme.colorScheme.errorContainer,
+                  ),
+                );
+              }
+              return;
+            }
 
             await ref.read(LaunchHelper.provider).launchUrl(codeResponse.url);