Fix Visual and Error

* add term highlights for Visual and VisualNOS
* make Error bold in gui

Signed-off-by: Jani Nurminen <slinky@iki.fi>
Deprecating the version string

At v2.25 (this version) the version string present in zenburn.vim is
deprecated since it's a bother to keep a hardcoded version string in the
file. Going forward, the versioning will use annotated tags instead.

Signed-off-by: Jani Nurminen <slinky@iki.fi>