The parsing for a `GTK_TYPE_SHORTCUT_ACTION` on a GtkShortcut in a
builder file can fail, resulting in a `NULL` GtkShortcutAction. This
currently does not result in any warnings, potentially making typos in
builder files tricky to debug.

Since `gtk_shortcut_action_parse_builder()` already sets an error if it
fails, we can just use that and simply return false.