Add proper error message when adding several files to a non-archive format such as bzip or gzip

Running `ouch c src/ t.bz` currently yields in
```
thread 'main' panicked at 'internal error: entered unreachable code', src/commands.rs:165:18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

We once had an error message for this. We should bring it back.