nbd: Quit server after any write error

We should never ignore failure from nbd_negotiate_send_rep(); if
we are unable to write to the client, then it is not worth trying
to continue the negotiation.  Fortunately, the problem is not
too severe - chances are that the errors being ignored here (mainly
inability to write the reply to the client) are indications of
a closed connection or something similar, which will also affect
the next attempt to interact with the client and eventually reach
a point where the errors are detected to end the loop.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1463006384-7734-4-git-send-email-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>