* Append an appropriate error message to `buf` following the failure
* of `hold_lock_file_for_update()` to lock `path`. `err` should be the
* `errno` set by the failing call.
* Append an appropriate error message to `buf` following the failure
* of `hold_lock_file_for_update()` to lock `path`. `err` should be the
* `errno` set by the failing call.