Define `[[ErrorData]]`

[legendecas]

https://tc39.es/ecma262/#sec-properties-of-error-instances defines that [[ErrorData]] is an internal slot whose value is undefined, used for identification only.

However, the [[ErrorData]] internal slot is used to get stack frames in https://tc39.es/proposal-error-stacks/#sec-getstackframes. I think this internal slot needs to be defined, as like "host defined", rather than undefined value.

[ljharb]

You're exactly right; that's a clarification I'd work on during stage 2.