[Nbd] more user-friendly error message

[Milos J. <xj...@fi...>]

Hi folks,

we recently had a bug filed against the nbd Fedora package claiming 
about the clarity of the error message which is printed when the nbd 
module is not loaded:

https://bugzilla.redhat.com/show_bug.cgi?id=496751

What about merging a tiny patch I attached?

Regards,
Milos