jupyter: if user attempts to "download notebook as PDF" and it fails, suggest the workaround-that-always-works -- "download your file as HTML instead of PDF and then, if you need it, you can convert HTML to PDF in your browser."

[williamstein]

There are several methods to export a Jupyter notebook to PDF, but they can all fail for a variety of reasons, which are not bugs in CoCalc, e.g., latex processing errors, nbconvert problems, using too much memory, etc. When this happens, the modal should suggest using HTML instead, with a link to open the html version. I think this approach always works.