cmd/compile/internal/noder,go/internal/gcimporter: return an error if not an archive file
Return an error from FindExportData variants if the contents are not
an archive file.
Change-Id: I2fa8d3553638ef1de6a03e2ce46341f00ed6965f
Reviewed-on: https://go-review.googlesource.com/c/go/+/626697
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Robert Griesemer <gri@google.com>
Commit-Queue: Tim King <taking@google.com>