MathJax fails on entities

Created by: fast-reflexes

Issue Summary

When MathJax encounters a non-existing entity, it tries to download a file that does not exist and then fails. I don't expect MathJax to be able to handle non-existing entities, but the error is still odd (should produce MathProcessing error or something instead). Is this behaviour expected and desired?

Steps to Reproduce:

Check code pen https://codepen.io/fast-reflexes/pen/LYbabaY and check the console (the actual browser console) for an error message when MathJax tries to download non-existing file https://cdnjs.cloudflare.com/ajax/libs/mathjax/3.1.2/es5/util/entities/f.js when presented with non-existing entity &fpp;.

Any other information you want to share that is relevant to the issue being reported. Especially, why do you consider this to be a bug? What do you expect to happen instead?

Technical details:

• MathJax Version: 3.1.2
• Client OS: Mac OS Catalina 10.15.7
• Browser: Chrome Version 88.0.4324.192 (Official Build) (x86_64)