nbd/client: Fix error messages during NBD_INFO_BLOCK_SIZE

nbd/client: Fix error messages during NBD_INFO_BLOCK_SIZE

A missing space makes for poor error messages, and sizes can't
go negative.  Also, we missed diagnosing a server that sends
a maximum block size less than the minimum.

Fixes: 081dd1fe
CC: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180501154654.943782-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
(cherry picked from commit e475d108f1b3d3163f0affea67cdedbe5fc9752b)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>