Fix showing success message if PR force-merge fails. (#1016)

Author: tech4him1

src/backends/github/API.js

@@ -638,7 +638,7 @@ export default class API {
     })
     .catch((error) => {
       if (error instanceof APIError && error.status === 405) {
-        this.forceMergePR(pullrequest, objects);
+        return this.forceMergePR(pullrequest, objects);
       } else {
         throw error;
       }