Wire up cred helper stderr

[jonjohnsonjr]

Fixes #528

This is much more useful:

$ crane push busybox.tar gcr.io/jonjohnson-test/tarballs/busybox:docker
Reauthentication required.
ERROR: (gcloud.auth.docker-helper) There was a problem reauthenticating while refreshing your current auth tokens: Reauthentication challenge could not be answered because you are not in an interactive session.
Please retry your command or run:

  $ gcloud auth login

To obtain new credentials.
2019/09/16 08:22:54 pushing gcr.io/jonjohnson-test/tarballs/busybox:docker: Get https://gcr.io/v2/token?scope=repository%3Ajonjohnson-test%2Ftarballs%2Fbusybox%3Apush%2Cpull&service=gcr.io: invoking docker-credential-gcloud: exit status 1; output: 

[codecov-io]

Codecov Report

Merging #529 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #529      +/-   ##
==========================================
+ Coverage   73.19%   73.19%   +<.01%     
==========================================
  Files          94       94              
  Lines        4006     4007       +1     
==========================================
+ Hits         2932     2933       +1     
  Misses        703      703              
  Partials      371      371

Impacted Files	Coverage Δ
pkg/authn/helper.go	90.9% <100%> (+0.43%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 164ee21...5e37fd8. Read the comment docs.