dbus: Hint that maybe squeekboard is running

It seems that is Squeekboard is already running, the next instance will not fail to acquire a bus name, but instead lose it immediately. This message has been reported by users who experiment with Squeekboard for the first time, so let's make it easier for them to find the solution without having to reach out.

src/server-main.c

@@ -82,7 +82,7 @@ on_name_lost (GDBusConnection *connection,
     (void)connection;
     (void)name;
     (void)user_data;
-    g_error("DBus unavailable, unclear how to continue.");
+    g_warning("DBus unavailable, unclear how to continue. Is Squeekboard already running?");
     exit (1);
 }