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

[Richard Kimberly Heck]

On 12/4/20 12:23 PM, Pavel Sanda wrote:
> 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.

Looks good.