[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/gcc.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/bash -e -o pipefail
 
 echo "Installing GCC@8 using homebrew..."
 brew install gcc@8