Fully implemented the fallback feature, using a second Cache

Changed to use a second Cache for fallback data.

Possible controversial decision: During the process, I removed the absoluteUrl function from the worker. I couldn't figure out the use case, and hence didn't know when to use it. For consistency I didn't use it anywhere.
Does this end up triggering the outer .catch(), leading to the fallback cache being tried?

(I'm a little shaky on that Promise syntax.)