Arvados

This was reported in #14804 but (judging by the code) the resulting change addressed only the specific example of Azure returning 503.

There are many other backend failure modes where "block not found" is the wrong response, like:

• Azure returns 502, 418, etc. or times out
• S3 returns 5xx
• Filesystem returns EPERM

Drivers are expected to return errors that satisfy os.IsNotExist() when they determine that a block does not exist. It seems like the logic should be:

• if all backends return 404 (os.IsNotExist(err)==true), then return 404
• otherwise -- i.e., when other errors prevent us from determining whether the block exists -- return 502