nbd/client: Fix error message for server with unusable sizing

Add a missing space to the error message used when giving up on a
server that insists on an alignment which renders the last few bytes
of the export unreadable.

Fixes: 3add3ab7
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190404145226.32649-1-eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>