localCI: improve message errors when the git command fails
localCI: improve message errors when the git command fails

this patch includes git's stderr in the error returned by
downloadPullRequest

fixes #184

Signed-off-by: Julio Montes <4a43a13347431ef00be7d745d08a5ba1757e840b@intel.com>

sboeuf/tests,bwarden/tests,clearcontainers/tests,MarioCarrilloA/tests,clearcontainers/tests,chavafg/tests,GabyCT/tests,devimc/tests,MarioCarrilloA/tests,bwarden/tests,sboeuf/tests,chavafg/tests,devimc/tests,GabyCT/tests