Handle errors in new NFS server creation gracefully

I often see errors like:

```
opam@7641e4cbcf64:/coclo$ ./disco.sh add-new-nfs tavi-cloud-nfs-16 2000 /nfs-pool-16
mount.nfs: access denied by server while mounting tavi-cloud-nfs-16:/nfs-pool
```

I then need to manually mount, add the server to `$NFS_MOUNTS` in `configuration.env`, and add the witness file. These steps can be automated.