* Recover after feed fetch failure with exponential backoff
The current implementation stops attempting to fetch a feed if fusion encounters any error fetching it. The only way to continue fetching the feed is if the user manually forces a refresh.
This allows fusion to recover from feed fetch errors by tracking the number of consecutive failures and slowing down requests for consistent failure. If a feed always fails, we eventually slow to only checking it once per week.
Fixes#67
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