Reverts the following changes:

  https://go.googlesource.com/go/+/1f90d081391d4f5911960fd28d81d7ea5e554a8f
  https://go.googlesource.com/go/+/8bf18b56a47a98b9dd2fa03beb358312237a8c76
  https://go.googlesource.com/go/+/5402854c3557f87fa2741a52ffc15dfb1ef333cc
  https://go.googlesource.com/go/+/37f84817247d3b8e687a701ccb0d6bc7ffe3cb78
  https://go.googlesource.com/go/+/6be6f114e0d483a233101a67c9644cd72bd3ae7a

Partially reverts the followinng change, removing the errors.Opaque
function and the errors.Wrapper type definition:

  https://go.googlesource.com/go/+/62f5e8156ef56fa61e6af56f4ccc633bde1a9120

Updates documentation referencing the Wrapper type.

Change-Id: Ia622883e39cafb06809853e3fd90b21441124534
Reviewed-on: https://go-review.googlesource.com/c/go/+/176997
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>