Function error_chain 

pub fn error_chain(e: &dyn StdError) -> String

Flatten an error’s source() chain into one ; caused by: string.

Storage and HTTP backends wrap transport errors several layers deep; the outer Display can show “error sending request” while the actual cause (e.g. rustls UnknownIssuer, hyper connect failure) is only reachable via source(). Logging the full chain at the failure site makes these self-diagnosing without per-crate trace logging.