[LyX/master] Improve VCS error message when RCS is not installed.

[Pavel Sanda]

On Fri, Dec 04, 2020 at 05:51:55PM +0100, Pavel Sanda wrote:
> commit b670990bc12fe1dd1d9d7b5fa8258a32caa7e91a
> Author: Pavel Sanda <sanda at lyx.org>
> Date:   Fri Dec 4 18:16:44 2020 +0100
> 
>     Improve VCS error message when RCS is not installed.
>     
>     User's report that it takes time to realize what package
>     is 'ci' part of, so we can give a hint for the initial
>     file registration.

Riki, this is trivial branch candidate. Pavel