Improve warning handling a bit

We were printing the full program name as prefix for warnings,
which does not really help with readability. Lets just use
the single-char 'F:' prefix for all messages, regardless of
log level.

Closes: #2472
Approved by: matthiasclasen

app/flatpak-main.c

@@ -178,11 +178,7 @@ message_handler (const gchar   *log_domain,
                  const gchar   *message,
                  gpointer       user_data)
 {
-  /* Make this look like normal console output */
-  if (log_level & G_LOG_LEVEL_DEBUG)
-    g_printerr ("F: %s\n", message);
-  else
-    g_printerr ("%s: %s\n", g_get_prgname (), message);
+  g_printerr ("F: %s\n", message);
 }
 
 static GOptionContext *