Display custom error messages from plugins hooks (as documented in the API spec)

index.php

@@ -147,7 +147,7 @@ if ($RCMAIL->task == 'login' && $RCMAIL->action == 'login') {
             $error_code = RCMAIL::ERROR_INVALID_REQUEST;
         }
         else {
-            $error_code = $auth['error'] ? $auth['error'] : $RCMAIL->login_error();
+            $error_code = is_numeric($auth['error']) ? $auth['error'] : $RCMAIL->login_error();
         }
 
         $error_labels = array(
@@ -157,7 +157,7 @@ if ($RCMAIL->task == 'login' && $RCMAIL->action == 'login') {
             RCMAIL::ERROR_INVALID_HOST     => 'invalidhost',
         );
 
-        $error_message = $error_labels[$error_code] ? $error_labels[$error_code] : 'loginfailed';
+        $error_message = !is_numeric($auth['error']) ? $auth['error'] : ($error_labels[$error_code] ?: 'loginfailed');
 
         $OUTPUT->show_message($error_message, 'warning');