src: rename unexported errors by adding prefix err

src: rename unexported errors by adding prefix err

By convention, use `err` as prefix for variables of type `error`.

Change-Id: I9401d5d47e994a27be245b2c8b1edd55cdd52db1
Reviewed-on: https://go-review.googlesource.com/c/go/+/467536
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>