Unhelpful errror message when writing to virtiofs isn't possible.

When (absolutely on purpose, of course) setting the `--readonly` flag on virtiofsd, and trying to write to a virtiofs mount, we get a rather unhelpful error message:
```
thread 'main' (1) panicked at src/main.rs:15:49:
called `Result::unwrap()` on an `Err` value: Kind(Uncategorized)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

The fuse response header includes the error code `-30` (Read-only filesystem), but apparently that is not converted correctly here.