Improve error handling in labextension install (#10233)
Improve error handling in labextension install (#10233)

* Get rid of unused variables in _get_labextension_metadata

* Report exceptions occurring while looking for labextensions

* Make labextension package discovery logic DRYer
jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab,jupyter/jupyterlab