nbd-server: use one error domain for all NBD server errors

nbd-server: use one error domain for all NBD server errors

The purpose of the commit is to move all error identifiers to one
namespace common for all errors originating from NBD server. Using
multiple internal error domains would add only bureaucracy without any
real gain. Error domains should be used to separate errors between
modules, not within modules. For example, NBD client would define its
own error domain and codes.

Signed-off-by: Tuomas Räsänen <tuomasjjrasanen@tjjr.fi>