Perform safety checks in create-stack-user.sh

This adds some safety checks to the stack user creation script.

This includes:
- Using set -o errexit to exit early on errors
- Make sure STACK_USER is set before doing anything with it

Change-Id: If027daddd03e32c5ba3c2ebb05ad5b27d2868b0a
Merge "Perform safety checks in create-stack-user.sh"