Add paragraph about browser add-ons when encountering some errors (#14801)

* Add paragraph about browser add-ons when encountering some errors

When a crash is caused by a NotFoundError exception, add a paragraph
to the error page mentioning browser add-ons.

Indeed, crashes with NotFoundError are often caused by browser extensions
messing with the DOM in ways React.JS can't recover from (e.g. issues #13325
and #14731).

* Reword error messages