Respect the raise_config_file_errors trait and re-raise if applicable.

[ian-r-rose]

I was taking a look at jupyterlab/jupyterlab#5716, and found that setting the base Application trait raise_config_file_errors wasn't behaving as I expected. This is because JupyterApp overloads the load_config_file function and doesn't check this option.

A quick audit of the Jupyter ecosystem shows that there are a couple of different behaviors around this.

• jupyterhub inherits from Application rather than JupyterApp, so the trait works as expected.
• IPython overrides load_config_file, and specifically checks for this option.
• Notebook and JupyterLab inherit from JupyterApp, and they do not respect this option.

Another option would be to override the load_config_file option in lab/notebook, but it seemed cleaner to me to add this check here.

[ian-r-rose]

Failure is due to setuptools no longer supporting python 3.3 (as of June 2018).

[ian-r-rose]

Ping @takluyver, any thoughts on this?

[takluyver]

Thanks, this looks fine, just one minor suggestion for code clarity.

[takluyver]

Thanks

[ian-r-rose]

Thanks for the review + merge!