strconv: add Unwrap to custom error types
Updates #30322
This change adds the Unwrap method to NumError. NumError is the only custom error type of the strconv that has a nested exported error.
Change-Id: I8774886348880365a83f72a1d106276def27dffe
GitHub-Last-Rev:
712f3df8842f48f988cebfc527476781a7cf7140
GitHub-Pull-Request: golang/go#34213
Reviewed-on: https://go-review.googlesource.com/c/go/+/194563
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>