Do not comment twice if backporting fails due to merge conflicts. (#15050)

Author: teo-tsirpanis

.github/workflows/backport-base.yml

@@ -152,7 +152,8 @@ jobs:
                 issue_number: pr_number,
                 body: git_am_failed_body
               });
-              throw new Error("Error: git am failed, most likely due to a merge conflict.", false);
+              core.setFailed("Error: git am failed, most likely due to a merge conflict.");
+              return;
             }
             else {
               // push the temp branch to the repository