xerrors: copied files from x/exp/errors

changed paths and removed experimental notice

Also made panic test pass: the output differed
for different Go versions.

Change-Id: Iadd367fb253459d288c3ec7e3aaafce9af939983
Reviewed-on: https://go-review.googlesource.com/c/159777
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
16 files changed