Exit out when an unsupported stack is used [#164076510]
Exit out when an unsupported stack is used [#164076510]

Co-authored-by: Guillermo Kardolus <f995e5708409e0468f92322e706c77752cc6c0d4@pivotal.io>

cloudfoundry/python-buildpack,cloudfoundry/python-buildpack,cloudfoundry/python-buildpack