Updated error messages for volume clone

Updated error messages for volume clone from image,
snapshot and volume to be consistent across.
Updated error messages to indicate the unit (GB).

Change-Id: I9f51ad0c9ce40864a838fca06583d7d4283e61d8
Closes-Bug: #1244609
Merge "Updated error messages for volume clone"
(Test commit) Change README to reflect Jio's fork of Cinder

Change-Id: I3f26c0a736e63725aec24ca2fa12c707b8e6460f