> But as a standard library abstraction, it’s too opinionated. It categorically excludes cases where sources form a tree: a validation error with multiple field failures, a timeout with partial results. These scenarios exist, and the standard trait offers no way to represent them.

This seems akin to complaining that the CPU core has only one instruction pointer. There is nothing preventing a struct implementing `Error` from aggregating other errors (such as validation results) and still exposing them via the `Error` trait. The fact of the matter is that the call stack is linear, so the interior node in the tree the author wants still needs to provide the aggregate error reporting that reflects the call stack that was lost with the various returns. Nothing about that error type implementing `Error` prevents it from also implementing another error reporting trait that reflects the aggregate errors in all of the underlying richness with which they were collected.