display Unexpected error: {exception message} instead of Unknown error upon login

lib/main/localizations/app_localizations.dart

@@ -114,6 +114,12 @@ class AppLocalizations {
         name: 'unknownError');
   }
 
+  String unexpectedError(String errorMessage) {
+    return Intl.message('Unexpected error: $errorMessage',
+        name: 'unexpectedError',
+        args: [errorMessage]);
+  }
+
   String get search_folder {
     return Intl.message(
       'Search folder',

lib/main/utils/toast_manager.dart

@@ -50,7 +50,11 @@ class ToastManager {
     } else if (exception is NoNetworkError) {
       return AppLocalizations.of(context).youAreOffline;
     } else {
-      return null;
+      try {
+        return AppLocalizations.of(context).unexpectedError(exception.message);
+      } catch(_) {
+        return AppLocalizations.of(context).unexpectedError(exception.toString());
+      }
     }
   }