fmt: make type of fmt.Errorf the same as that of errors.New

This applies only for cases where %w is not used.

The purpose of this change is to reduce test failures where tests
depend on these two being the same type, as they previously were.

Change-Id: I2dd28b93fe1d59f3cfbb4eb0875d1fb8ee699746
Reviewed-on: https://go-review.googlesource.com/c/go/+/167402


Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>