Make list_patch_packages non fatal for RunTier0Tests.py
Since we updated atlasexternals to 2.0.28, we no longer write out ${WorkDir_DIR}/packages.txt
that lists which packages are locally configured/compiled. As a result, RunTier0Tests.py was crashing. This merge request simply puts this step in a try block so that it's no longer a fatal problem. Instead, we print a warning message.