Say in a failed economy answer how long its failure is kept #4171

A failed economy calculation's 502 body is {"status": "error", "message": "...", "result": null}. When a new run can be requested is written only in the message, for example "A new run for this state can be requested 2 minutes after this failure." (#4064). A client that wants to know when to offer Retry has to parse that sentence, which PolicyEngine/policyengine-app-v2#1439 does today.

The stored failure already has an expiry in the runtime cache: 2 minutes for a failed US state or local-area breakdown job, and 30 days for every other failure, including US state failures stored before #4064. Until it expires, every request gets the same failure, and the first request after it submits a new job.

The 502 body should carry that remaining time as a number, read from the cache's own expiry, so clients read a field instead of the message text.