Simplify error finding and help displaying functions.

* tex-buf.el (TeX-parse-TeX): Use `TeX-find-display-help' in place
of `TeX-error-list-find-display-help'.
(TeX-error-list-find-display-help): Removed, replaced by more
general `TeX-find-display-help'.
(TeX-find-display-help): New function.
(TeX-error): Append nils to the `TeX-error-list' entry to make
each entry of the same lenght for both errors and warnings.
Append also `TeX-error-point'.  This fixes a bug occurring when
`TeX-display-help' is set to `expert'.  Use
`TeX-find-display-help' to display the help.
(TeX-warning): Append `TeX-error-point' to the `TeX-error-list'
entry to fix the above mentioned bug.  Use `TeX-find-display-help'
to display the help.
error overview
open error overview in a new frame
add documentation
replace functionp with fboundp
fix
simplify error finding and help displaying functions
fix
document navigation in error overview in the manual