The SDK retries a request that fails before its response starts, but not a
block body cut off part way through. That error stopped playback or failed
a download, and the SDK's new keep-alive pings make it the way a dead
connection now ends: seconds in, rather than at the storage timeout.
A block is now fetched up to three times, 250 ms and then 500 ms apart,
while ProtonError::is_retriable says a replay could succeed. A permanent
refusal still fails on the first attempt.
nvrl