Improve log message when fgfs_lock.pid exists

Part of addressing:
https://sourceforge.net/p/flightgear/codetickets/2133/

Still need to decide about a better GUI dialog for this.

src/Main/fg_init.cxx

@@ -515,6 +515,8 @@ bool fgInitHome()
         if (err < 0) {
             if ( errno ==  EWOULDBLOCK) {
                 SG_LOG(SG_GENERAL, SG_ALERT, "flightgear instance already running, switching to FG_HOME read-only. ");
+                SG_LOG(SG_GENERAL, SG_ALERT, "Couldn't flock() file at:" << pidPath);
+
                 // set a marker property so terrasync/navcache don't try to write
                 // from secondary instances
                 fgSetBool("/sim/fghome-readonly", true);