Force build_image.sh to exit 1 when fetch_subprojects fails
Force build_image.sh to exit 1 when fetch_subprojects fails

fetch_subproject script is called via a new shell instead
of all other sourced functions that can force to exit the build
script.

This patch catch the result of fetch_subprojects script and exit the
build_script if needed.

Change-Id: I0ddca025e00fa5911682cf8faba35df25e28176f

enovance/software-factory,enovance/software-factory,enovance/software-factory,enovance/software-factory,enovance/software-factory