Write errors to stderr to get git to abort and show them as such.
Addresses private issues https://dev.gitlab.org/gitlab/gitlab-shell/issues/33 and https://dev.gitlab.org/gitlab/gitlabhq/issues/2195.
Addresses private issues https://dev.gitlab.org/gitlab/gitlab-shell/issues/33 and https://dev.gitlab.org/gitlab/gitlabhq/issues/2195.