Fail stack-create if script exits with non-zero status

If the heat-script or cfn-userdata script exits with a non-zero
status, the user will see which script failed and where to find
the debug log on the server.  This will allow the template author to
conditionally fail a stack-create in the user_data section.

Change-Id: Ibc3768d8192102b9057d4ba873058cb50abcd1b1
Merge "Fail stack-create if script exits with non-zero status"