Disallow quota deletes if default under usage

Disallow quota deletes if default under usage

For nested quotas, currently we don't allow the limit to be updated
below the current usage since this could make a portion of the
quota tree "invalid". This is explicitly disallowed for quota update
but not currently checked on quota delete. This patch adds the same
checking behavior for quota-deletes. Also, the quota delete
authentication check is being moved above the validation code, so that
a user not allowed to perform a delete will always see an unauthorized
error instead of any other invalid errors with the request.

Change-Id: I390ffa38b6c03c034614756ca410dfca9e52f2ce
Closes-Bug: #1555802