Comment 11 for bug 1118066

I would propose the following behavior:

When os-quota-sets is updated, nova-api checks the quota tables to see if the quota-set for the project ID already exists in the table. If it does exist, then update with the new quota value. Otherwise, use keystoneclient to confirm that the project ID exists. If it does not exist, return an appropriate error to the API. Otherwise update the new quota value.

This will catch the error except for cases where the quota table is already corrupted with quotas that apply to no projects.