Akkoma (and Pleroma?) do not add a X-Ratelit-Reset header (or anything
that would give insight as to the state of the rate limiter) to their
429 errors.
This currently makes FediFetcher raise a KeyError, notably on fetching
the timeline of an Akkoma server.

This works around it by falling back to an internally provided wait
time. It starts at half a second, and is mulitplied by 4 on each retry. For
the default of 5 retries this should wait a total of ~3 minutes.
This should cover aggressively configured rate-limiters as well as
handling the default Akkoma timeline limit almost optimally (which
replenishes every half a second:
https://akkoma.dev/AkkomaGang/akkoma/src/commit/e326285085e40c8a75f5c154122052d371a144a1/config/config.exs#L727)