Feature request: Tell user that "_xsrf" warnings mean saving didn't happen

I'd like to request a partial (hopefully easy) fix for an issue that's been reported before.

Sometimes the server has trouble communicating with the notebook page and a warning comes up. This warning has to do with some `_xsrf` field, and appears to mean that saving the notebook has failed. This doesn't seem that uncommon:

* https://stackoverflow.com/questions/55014094/jupyter-notebook-not-saving-xsrf-argument-missing-from-post
* https://github.com/jupyter/notebook/issues/4495
* https://github.com/jupyter/notebook/issues/4092

Instead of having a little yellow message pop up in the logs and notebook header that says "'_xsrf' argument missing from POST", could there be a big red message that says "Something went wrong with saving"? Cause it really sucks when you lose a few hours of work late in the day.

It sounds like jupyter lab might already have this warning: https://github.com/jupyterlab/jupyterlab/issues/3506.

Start by reading the prior reports in issues 4495 and 4092 and the related JupyterLab issue 3506, then trace where the `_xsrf` warning appears in the notebook page, logs, and header. Done means a prominent red message clearly tells users that saving failed when this warning occurs.

Written by the indexing model from the issue text.