Replaced calls to os._exit(0) in no_notebook with graceful shutdown
Replaced calls to os._exit(0) in no_notebook with graceful shutdown

mwcraig/vpython-jupyter,mwcraig/vpython-jupyter,BruceSherwood/vpython-jupyter,BruceSherwood/vpython-jupyter,BruceSherwood/vpython-jupyter,mwcraig/vpython-jupyter,BruceSherwood/vpython-jupyter,mwcraig/vpython-jupyter