merge_into_manifest: fix error message if MR already exists

merge_into_manifest.py

@@ -61,7 +61,7 @@ def merge_into_manifest(
         manifest_project, integration_branch, target_branch
     )
     if not created:
-        sys.exit("ERROR: There is already an open merge request:\n%s" % mrs[0].web_url)
+        sys.exit("ERROR: There is already an open merge request:\n%s" % mr.web_url)
 
     print("Created new merge request:")
     print(mr.web_url)