cmd/go: show full path name (testdata/script/foo.txt) in script errors

cmd/go: show full path name (testdata/script/foo.txt) in script errors

Showing the full path (relative to the current directory)
instead of just foo.txt lets editors that understand file:line
jump straight to the file without having to edit it to say
testdata/script/ first.

Change-Id: I44177b687249f3c7c724b45d02f5167607369e1b
Reviewed-on: https://go-review.googlesource.com/c/go/+/498119
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>