cmd/go: handle error when git remote origin doesn't exist

cmd/go: handle error when git remote origin doesn't exist

- Let runOutput return the error message
- When `git config ...` returns empty buffer, it means the config key is
  correct, but there is no corresponding value.
- Return the correct error when the url of remote origin is not found.
- Update error message

Fixes: #10922
Change-Id: I3f8880f6717a4f079b840d1249174378d36bca1b
Reviewed-on: https://go-review.googlesource.com/10475
Reviewed-by: Russ Cox <rsc@golang.org>