gen_release_notes: stop the script if we can't generate the release note

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20459>

bin/gen_release_notes.py

@@ -343,6 +343,7 @@ async def main() -> None:
             ))
         except:
             print(exceptions.text_error_template().render())
+            return
 
     subprocess.run(['git', 'add', final])
     subprocess.run(['git', 'commit', '-m',