Change error message prefix from "tar:" to "archive/tar:" to maintain
backwards compatibility with Go1.9 and earlier in the unfortunate event
that someone is relying on string parsing of errors.

Fixes #22740

Change-Id: I59039c59818a0599e9d3b06bb5a531aa22a389b8
Reviewed-on: https://go-review.googlesource.com/77933Reviewed-by: roger peppe <rogpeppe@gmail.com>