Add error handler for ld-linux-x86-64.so.2 not found

This happens when the user is trying to run a Linux application, `linux64.ko` is loaded but no `/compat` is mounted and we don't have a fully self-contained bundle:

Currently no error dialog is shown at all.

```
FreeBSD% launch '/home/user/Applications/Linuxulator/Spotify.AppDir'
# Found "/home/user/Applications/Linuxulator/Spotify.AppDir"
ELF interpreter /lib64/ld-linux-x86-64.so.2 not found, error 2
Process is not running anymore and exit code was not 0
FreeBSD% launch '/home/user/Applications/Linuxulator/Discord.app'
# Found "/home/user/Applications/Linuxulator/Discord.app"
ELF interpreter /lib64/ld-linux-x86-64.so.2 not found, error 2
Process is not running anymore and exit code was not 0
```

Maybe we should point the user toward a download link for the Linux userland/runtime.