Improve error message

lib/api/api_util.dart

@@ -6,6 +6,7 @@ import 'package:logging/logging.dart';
 import 'package:nc_photos/account.dart';
 import 'package:nc_photos/api/api.dart';
 import 'package:nc_photos/entity/file.dart';
+import 'package:nc_photos/exception.dart';
 
 /// Return the preview image URL for [file]. See [getFilePreviewUrlRelative]
 String getFilePreviewUrl(
@@ -62,9 +63,15 @@ Future<String> exchangePassword(Account account) async {
     },
   );
   if (response.isGood) {
-    final appPwdRegex = RegExp(r"<apppassword>(.*)</apppassword>");
-    final appPwdMatch = appPwdRegex.firstMatch(response.body);
-    return appPwdMatch.group(1);
+    try {
+      final appPwdRegex = RegExp(r"<apppassword>(.*)</apppassword>");
+      final appPwdMatch = appPwdRegex.firstMatch(response.body);
+      return appPwdMatch.group(1);
+    } catch (_) {
+      // this happens when the address is not the base URL and so Nextcloud
+      // returned the login page
+      throw InvalidBaseUrlException();
+    }
   } else if (response.statusCode == 403) {
     // If the client is authenticated with an app password a 403 will be
     // returned

lib/bloc/app_password_exchange.dart

@@ -3,6 +3,7 @@ import 'package:flutter/foundation.dart';
 import 'package:logging/logging.dart';
 import 'package:nc_photos/account.dart';
 import 'package:nc_photos/api/api_util.dart' as api_util;
+import 'package:nc_photos/exception.dart';
 
 abstract class AppPasswordExchangeBlocEvent {
   const AppPasswordExchangeBlocEvent();
@@ -72,6 +73,9 @@ class AppPasswordExchangeBloc
     try {
       final appPwd = await api_util.exchangePassword(account);
       yield AppPasswordExchangeBlocSuccess(appPwd);
+    } on InvalidBaseUrlException catch (e) {
+      _log.warning("[_exchangePassword] Invalid base url");
+      yield AppPasswordExchangeBlocFailure(e);
     } catch (e, stacktrace) {
       _log.shout("[_exchangePassword] Failed while exchanging password", e,
           stacktrace);

lib/exception.dart

@@ -45,3 +45,19 @@ class PermissionException implements Exception {
 
   final dynamic message;
 }
+
+/// The Nextcloud base URL address is invalid
+class InvalidBaseUrlException implements Exception {
+  InvalidBaseUrlException([this.message]);
+
+  @override
+  toString() {
+    if (message == null) {
+      return "InvalidBaseUrlException";
+    } else {
+      return "InvalidBaseUrlException: $message";
+    }
+  }
+
+  final dynamic message;
+}

lib/exception_util.dart

@@ -16,6 +16,8 @@ String toUserString(dynamic exception, BuildContext context) {
     }
   } else if (exception is SocketException) {
     return AppLocalizations.of(context).errorDisconnected;
+  } else if (exception is InvalidBaseUrlException) {
+    return AppLocalizations.of(context).errorInvalidBaseUrl;
   }
   return exception.toString();
 }

lib/l10n/app_en.arb

@@ -379,5 +379,9 @@
 	"errorLocked": "File is locked on server. Please try again later",
 	"@errorLocked": {
 		"description": "Error message when server responds with HTTP423"
+	},
+	"errorInvalidBaseUrl": "Unable to communicate. Please make sure the address is the base URL of your Nextcloud instance",
+	"@errorInvalidBaseUrl": {
+		"description": "Error message when the base URL is invalid"
 	}
 }