Also check for invalid ids in checkcellmetadata.py
Also check for invalid ids in checkcellmetadata.py

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