Fix error message when updating quota values
Fix error message when updating quota values

The error message mixes smaller and greater which leads to error
messages like "Quota value 4000 for gigabytes are greater than
already used and reserved 8000.". The proposed patch fixes this
(and replaces 'are' by 'is' in the message).

Change-Id: I90b04ae12235d5fabc57d549aff53eaa7bd99565

openstack/manila,openstack/manila