Array sort_custom should show a warning or error if the function doesn't exist.

[CowThing]

Godot version:
3.0 / 3.1

Issue description:
There should be a warning or error message when the sort_custom fails because of the function not existing, like in the example below. I had a misspelled function name and it took me a while to figure out why it wasn't sorting.

array.sort_custom(self, "non_existent_function")

[akien-mga]

Reopening as #22737 had to be reverted for now due to #22833 (which is a consequence of this patch, but due to the other bug #22838).