Warn/disallow entries for non-existing PPAs

[Michael Vogt]

It appears that software-center-agent allows to create archive_ids that are not actually existing, like e.g. "archive_id": "commercial-ppa-uploaders/skype". I understand this is useful for preparing apps that are not quite ready yet, but it would be good to warn or disallow this as it may result in apps that are marked for purchase but are not actually purchasable.

[Michael Vogt (mvo)]

In addition to that we should add a jenkins job to automatically scan that all archive_ids are actually accessable, i.e. talk to the API, look for each archive_id, GET on it and check 403 vs 404 so that we can trigger a alert if we have incorrect data in the API.