fix(git-push):handle the errors during git push

[kmala]

Summary of Changes

make sure the errors are not suppressed when output is piped allowing successive pushes without a dummy commit.

Issue(s) that this PR Closes

Associated End To End Test PR(s)

Associated Documentation PR(s)

Associated Design Document(s)

Testing Instructions

Please provide a detailed list for how to test the changes in this PR.

1. Create a Deis Cluster
2. deploy a app which errors out.
3. git push should fail saying remote rejected meaning you should be able to do another git push without a dummy commit.

   Pull Request Hygiene TODOs

Please make sure the below checklist is complete.

• [x] Your pull request is concise and to the point (make another PR for refactoring nearby code)
• [x] Your commits are squashed into logical units of work
• [x] Your commits follow the commit style guidelines

[arschles]

@kmala code LGTM. can you create an issue in deis/workflow-e2e to add an e2e test for this?

(I don't remember if I already asked you to do this. if I did, then ignore me)

[kmala]

e2e test for checking this https://github.com/deis/workflow-e2e/issues/185