Fix exit code in runtests script
Fix exit code in runtests script

MatKallada/nbgrader,ellisonbg/nbgrader,ellisonbg/nbgrader,modulexcite/nbgrader,alope107/nbgrader,jupyter/nbgrader,jdfreder/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,EdwardJKim/nbgrader,jhamrick/nbgrader,modulexcite/nbgrader,jupyter/nbgrader,jupyter/nbgrader,alope107/nbgrader,jupyter/nbgrader,dementrock/nbgrader,ellisonbg/nbgrader,jdfreder/nbgrader,EdwardJKim/nbgrader,jupyter/nbgrader,dementrock/nbgrader,ellisonbg/nbgrader,jhamrick/nbgrader,jhamrick/nbgrader,MatKallada/nbgrader,jhamrick/nbgrader