Add TeX-previous-error to Command menu.

* tex.el (TeX-mode-specific-command-menu-entries): Add
`TeX-previous-error'.

* tex-buf.el (TeX-error-list): Fix typo.
(TeX-parse-all-errors): Ditto.
Merge branch 'master' into simplify-TeX-parse-error
Color TeX Help buffer.
Fix possible infinite loop in TeX-parse-all-errors.

* tex-buf.el (TeX-warning): Preserve point when searching backward.  In
some cases this prevents infinite loops in `TeX-parse-all-errors' and
fixes wrong detection of context string.
Merge from master.