Cater for bad boxes in error descriptions.

* tex-buf.el (TeX-parse-TeX): Use
`TeX-error-list-find-display-help'.
(TeX-error-list-find-display-help): New function.
(TeX-warning): Use `bad-box' when there is a bad box.
(TeX-warning--find-display-help): Cater for bad boxes.
(TeX-help-error): Ditto.