gosxappinfo: Correctly return an error from create_from_commandline()

Creating a `GAppInfo` from a commandline isn’t currently supported on
macOS, but the implementation was incorrectly returning `NULL` without
setting the `GError`.

This was being caught by the new tests in `gio/tests/file.c`.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

gio/gosxappinfo.m

@@ -587,6 +587,9 @@ g_app_info_create_from_commandline (const char           *commandline,
                                     GAppInfoCreateFlags   flags,
                                     GError              **error)
 {
+  g_set_error_literal (error, G_IO_ERROR, G_IO_ERROR_NOT_SUPPORTED,
+                       "Creating an app info from a command line not currently supported");
+
   return NULL;
 }