extensions-tool: Handle existing extension folder

g_file_make_directory_with_parents() returns an error if the directory
already exists (unlike g_mkdir_with_parents()), so it is currently
impossible to install more than a single extension, whoops.

Handle the error properly by just carrying on.

https://gitlab.gnome.org/fmuellner/gnome-extensions-tool/merge_requests/3
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1234

src/extensions-tool/command-install.c

@@ -121,7 +121,12 @@ install_extension (const char *bundle,
                                       "gnome-shell", "extensions", NULL);
 
   if (!g_file_make_directory_with_parents (dstdir, NULL, &error))
+    {
+      if (error->code == G_IO_ERROR_EXISTS)
+        g_clear_error (&error);
+      else
         goto err;
+    }
 
   uuid = json_object_get_string_member (metadata, "uuid");
   dst = g_file_get_child (dstdir, uuid);