[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/vsmac.sh

@@ -1,4 +1,5 @@
-#!/bin/sh
+#!/bin/bash -e -o pipefail
+
 source ~/utils/utils.sh
 source ~/utils/xamarin-utils.sh
 
@@ -22,9 +23,6 @@ echo "Moving Visual Studio to /Applications/..."
 pushd $TMPMOUNT
 tar cf - "./Visual Studio.app" | tar xf - -C /Applications/
 
-echo "Launching vstools..."
-/Applications/Visual\ Studio.app/Contents/MacOS/vstool
-
 popd
 sudo hdiutil detach "$TMPMOUNT"
 sudo rm -rf "$TMPMOUNT"