glusterd/snapshot: Proper err msg for snapshot create command

problem: Snapshot command fails if one or more bricks are not thinly
provisioned. But the error message is a generic error message which
is confusing to the user.

fix: Provide correct error message in case of failure.

Change-Id: Iad247f966423a8f73ef6da57cab7ed6cddc05861
BUG: 1123646
Signed-off-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-on: http://review.gluster.org/8377
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>