PT: Fix KeyError while accessing API response

Traceback:
gitflow\pivotal.py”, line 143, in update_story
raise GitflowError(e.parsed_body[‘errors’][‘error’])
KeyError: ‘errors’

Fix:
Use e.parsed_body['message'] since that is the only field present.

See https://trello.com/c/sBbjTDqn for more details.