String.fromCodePoint panics on some code points

[aapoalas]

Unwrap panics at nova_vm/src/ecmascript/builtins/text_processing/string_objects/string_constructor.rs:255:60:. Presumably about invalid UTF-16, ie. WTF-16.

Eventually this will be fixed by us properly adopting WTF-8, but for now the method should rather throw an error instead of panicking.