Supress stderr from fetchrepo if successful (#590)
Supress stderr from fetchrepo if successful (#590)

Without this change, most repos while fetching will print messages like
this on stderr:
  .../go_repository.bzl:127:13: fetch_repo: go: finding google.golang.org/api v0.1.0
bazelbuild/bazel-gazelle,bazelbuild/bazel-gazelle