replaygain: Missing files with `ffmpeg` backend hang

### Problem

Remove some files from your library; then, without running `beet update`, use

```sh
beet replaygain
```

with the `ffmpeg` backend. You will get:

```
ffmpeg exited with status 254
```

and the program will hang. Instead, it should suggest `beet update` to correct for missing files, or detect and skip those (in which case it may be a more general problem rather than `ffmpeg`-specific).

### Setup

* OS: macOS 15
* Python version: 3.14.7
* beets version: 2.13.1
* Turning off plugins made problem go away (yes/no): N/A

My configuration (output of `beet config`) is:

```yaml
plugins:
- replaygain
replaygain:
auto: yes
backend: ffmpeg
```

Start by reproducing the hang with a removed library file using `beet replaygain` and the `ffmpeg` backend, then trace the replaygain command and backend handling of the `ffmpeg exited with status 254` result. Done means missing files no longer hang and the command either suggests `beet update` or safely detects and skips them.

Written by the indexing model from the issue text.