Add compatibility with Notebook 4.2 and add an error message for Notebook 4.3.0
Add compatibility with Notebook 4.2 and add an error message for Notebook 4.3.0

jupyter/jupyterlab,eskirk/jupyterlab,eskirk/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,charnpreetsingh185/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,eskirk/jupyterlab,charnpreetsingh185/jupyterlab,charnpreetsingh185/jupyterlab,eskirk/jupyterlab