[release-branch.go1.11] encoding/json: recover saved error context when unmarshalling

Fixes #29364

Change-Id: I270c56fd0d5ae8787a1293029aff3072f4f52f33
Reviewed-on: https://go-review.googlesource.com/132955


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit 22afb357)
Reviewed-on: https://go-review.googlesource.com/c/155377
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>