If we can't execute the go command, check to see if it exists. If it doesn't,
say so in the error.
Fixesgolang/go#29552
Change-Id: Iffd2db5fc8f1daa2e458eaa326b27d9a0d971b6b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/168777
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>