glusterd: Error msg for snapshot status for no existing snaps

glusterd: Error msg for snapshot status for no existing snaps
should be aligned with error messages of info and list

When a snapshot operation like status, info, list
performed on a non-existing snapshot.
For Status error message is displayed as 'Snap not found'
For List and Info error message is displayed as 'Snapshot does not exist'
Have the consistant error message all the places

Change-Id: I7b241217dba62fda844481731a6858e4ecb12897
BUG: 1119641
Signed-off-by: Vijaikumar M <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/8309
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>