Fix TeX-find-display-help.

* tex-buf.el (TeX-find-display-help): Expand the name of the file to be
visited starting from the directory of the master file.
Prettify file names in error overview.

* tex-buf.el (TeX-error-overview-make-entries): Add optional
`master-dir' argument, to shorten file names when they are relative.
(TeX-error-overview): Pass `TeX-master-directory' as argument to
`TeX-error-overview-make-entries'.