cmd/go: fix detection of unexpected files in downloaded zips
A bug in the old code was indirectly causing a confusing print,
but CL 131635 fixed the print instead of the surrounding code.
Fix the surrounding code, restore the old print, and test that the
error is actually reported (it was being ignored in a direct go get
but displaying in go build).
Change-Id: I03c21380fce481060c443b0cc820f3617497fdd9
Reviewed-on: https://go-review.googlesource.com/c/149317 Reviewed-by: Bryan C. Mills <bcmills@google.com>