An Http_Error with status 503, for when the service cannot answer now; add Retry-After with with_header.
format and args make the detail text as tprint does, in the temporary allocator.
assert(unavailable("down for maintenance").status == 503);Doesn't run in the browser (skipped on wasm).
unavailable :: (format := "", args: ..Any) -> Http_ErrorParameters
- format := ""
- args: ..Any