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>