Support 6645 - better error message for incomplete download as link

extension/qfq/qfq/report/Download.php

@@ -226,7 +226,8 @@ class Download {
 
         $arr = explode(':', $element, 2);
         if (count($arr) != 2) {
-            throw new DownloadException('Missing parameter for "' . $element . '"', ERROR_MISSING_REQUIRED_PARAMETER);
+            $possibleReason = ($element === '') ? 'If this is a download link, did you forget to include s:1?' : '';
+            throw new DownloadException("Missing parameter for '$element'. $possibleReason", ERROR_MISSING_REQUIRED_PARAMETER);
         }
 
         $token = $arr[0];