Fix wrong description when updating quotas

when updating quota for a resource, with 'force_update=false', an
exception will be raised when the new value is less than the used quota
values(used and reserved). This patch changes the description of
exception.

Change-Id: Iede70444341817afb6953eb604fe4de186622916
Closes-bug: #1243062
Merge "Fix wrong description when updating quotas"