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

[Pavel Sanda]

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.
---
 src/VCBackend.cpp |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/src/VCBackend.cpp b/src/VCBackend.cpp
index a1e0ec2..8f23927 100644
--- a/src/VCBackend.cpp
+++ b/src/VCBackend.cpp
@@ -58,11 +58,15 @@ int VCS::doVCCommand(string const & cmd, FileName const & path, bool reportError
 
 	if (owner_)
 		owner_->setBusy(false);
-	if (ret && reportError)
+	if (ret && reportError) {
+		docstring rcsmsg;
+		if (prefixIs(cmd, "ci "))
+			rcsmsg = "\n" + _("Perhaps the RCS package is not installed on your system?");
 		frontend::Alert::error(_("Revision control error."),
 			bformat(_("Some problem occurred while running the command:\n"
-				  "'%1$s'."),
+				  "'%1$s'.") + rcsmsg,
 			from_utf8(cmd)));
+	}
 	return ret;
 }