[macOS] Fail builds on errors during the image generation (#1756)

* set -e and fix all the scripts

* add source utils to finalize_vm script

* change xcode version in postbuild script

* fix for softwareupdates and for xcode version

images/macos/provision/core/python.sh

@@ -1,4 +1,5 @@
-#!/bin/sh
+#!/bin/bash -e -o pipefail
+
 source ~/utils/utils.sh
 
 echo "Installing Python Tooling"