Description of problem:

when we try to create a volume from a snapshot which is in error status we are getting the below error: 

ERROR: Invalid snapshot: status must be available

my first thought when seeing this was that the snapshot itself was not available and not that we are referring to the snapshot's status.

I think that if we change it to: 'snapshot's status must be available'
it would make the error message a clearer 

Version-Release number of selected component (if applicable):

openstack-cinder-2013.1.2-3.el6ost.noarch

How reproducible:

100%

Steps to Reproduce:
1. change openstack to work with external nfs storage 
2. create snapshot for a volume
3. create a volume from the snapshot

Actual results:

we fail to create the snapshot with the following error: 

ERROR: Invalid snapshot: status must be available

Expected results:

I think that if we add the word snapshot's before 'status must be available' it will make the error a bit clearer. 

Additional info: