nbd: Use return values instead of error_is_set(errp)

Using error_is_set(errp) to check whether a function call failed is
fragile: it breaks when errp is null.  Check perfectly suitable return
values instead when possible.  errp can't be null there now, but this
is more robust and more obviously correct

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
