fix: wrap error messages in fmt.Errorf

[jamescoombs-soda]

use %w instead of %v when nesting error messages so they can be properly unwrapped by upstream callers.

I've run into a problem where I'm unable to use errors.Is() in my tooling to help properly respond to errors coming out of oidc. The primary use case is checking for context.Canceled, but I'm sure there are others.

Note: I believe %w was introduced in Go version 1.13, so this change isn't friendly to users who use a 5 year old Go version but keep their packages up to date. In any case, it's possible this package already requires a higher Go version than that.

[ericchiang]

Thanks for the PR!

I'm against using wrapped errors overly-broadly like this without making it part of the API contract. At the very least, there needs to be an associated test, since users might depend on a method returning a specific error type.

You mentioned checking for context.Canceled. Do you have an example? Particularly with the new "Cause" API, there are other ways of handling context cancelation.

https://pkg.go.dev/context#Cause

Do you have a sense of what the godoc changes would be before we move forward with the code itself?