Fix GCS spec, revert back to original error message
Fix GCS spec, revert back to original error message

[#151699248](https://www.pivotaltracker.com/story/show/151699248)

Signed-off-by: Difan Zhao <d4b657a67385123ddb690ebdb0d79b9c6cd38927@pivotal.io>

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh