extensions-tool: Log existing errors

In many cases we currently only indicate failure in the return value,
which is easily missed. Print some meaningful errors instead.

https://gitlab.gnome.org/GNOME/gnome-shell/issues/2391

subprojects/extensions-tool/src/command-list.c

@@ -60,7 +60,10 @@ list_extensions (ListFilterFlags filter, DisplayFormat format)
                                      NULL,
                                      &error);
   if (response == NULL)
-    return FALSE;
+    {
+      g_printerr (_("Failed to connect to GNOME Shell\n"));
+      return FALSE;
+    }
 
   extensions = g_variant_get_child_value (response, 0);