It's poignant to watch Go slowly realize the why other languages have more powerful, more useful error types. The `error` interface is anemic to the point of uselessness. Making it unwrappable is step forward, but all that does is progress it the level of 1995 Java with their "cause" field. It still can't express suppressed errors (like those from `defer f.Close()`). There is no standard mechanism to include an optional stack trace. Worst of all, rolling your own error type and trying to use it everywhere is near impossible. Every method signature has to use `error` due to lack of covariant return types, and heaven help you if you return a nil struct pointer, which will be auto promoted to a non-nil interface. Perhaps in Go 3 they'll get it right.