[LyX/master] Tell the user that LyX is already running when we do not start because of 'use single instance'.

[Richard Kimberly Heck]

commit d55077477233fba7bb1154661634c4f005995707
Author: Richard Kimberly Heck <rikiheck at lyx.org>
Date:   Sat Apr 18 14:41:26 2020 -0400

    Tell the user that LyX is already running when we do not start
    because of 'use single instance'.
---
 src/Server.cpp |    6 ++++--
 1 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/src/Server.cpp b/src/Server.cpp
index ab1b636..5fb089d 100644
--- a/src/Server.cpp
+++ b/src/Server.cpp
@@ -1002,9 +1002,11 @@ struct Sleep : QThread
 
 bool LyXComm::loadFilesInOtherInstance()
 {
-	if (theFilesToLoad().empty())
+	if (theFilesToLoad().empty()) {
+		LYXERR0("LyX is already running in another instance\n"
+			"and 'use single instance' is active.");
 		return true;
-
+	}
 	int pipefd;
 	int loaded_files = 0;
 	FileName const pipe(inPipeName());