add better OAuth error

Fixes #88

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;
+}