testing/iotest: correct ErrReader signature and remove exported error

Corrects ErrReader's signature to what was accepted in the approved
proposal, and also removes an exported ErrIO which wasn't part of
the proposal and is unnecessary.

The new signature allows users to customize their own errors.

While here, started examples, with ErrReader leading the way.

Updates #38781

Change-Id: Ia7f84721f11061343cfef8b1adc2b7b69bc3f43c
Reviewed-on: https://go-review.googlesource.com/c/go/+/248898


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>