Handle "temporarily down" situations (http 503) more gracefully

This is an attempt at ensuring we handle situations where the remote service is down in an expected manner, and tells us this using the standard 503 http status when to try again.

An example of such a popup (with the text from the provider failing to load):

This further lays the groundwork for implementing handling of other http status situations (such as the annoying error 99, which happens occasionally during searches), but this one is functionally important as it allows us to temporarily take down the OCS servers and actually tell the users what is going on in a pleasant fashion, and even in some instances handle it automatically.

This requires attica!11 (merged)