cmd/cgo: report exec errors a bit more clearly

Change-Id: I0e6bebf0e2e6efdef4be880e0c6c7451b938924b
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/949417


Reviewed-by: Katie Hockman <katiehockman@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/284781


Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Roland Shoemaker <roland@golang.org>