Check for error conditions in promises, add execution tests.
Check for error conditions in promises, add execution tests.

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