execIO has multiple return paths and multiple places where error is
mangled. This CL simplifies the function by just having one return
path.
Some more tests have been added to ensure that the error handling
is done correctly.
Updates #19098.
Change-Id: Ida0b1e85d4d123914054306e5bef8da94408b91c
Reviewed-on: https://go-review.googlesource.com/c/go/+/662215
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Carlos Amedee <carlos@golang.org> Reviewed-by: Carlos Amedee <carlos@golang.org>