Improve “Illegal invocation” error messages

[ExE-Boss]

The improved error message format is based on the one used in Firefox.

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Related to #204 and #217.

[domenic]

This is a great idea. However, I'd like to avoid using spec-facing terms like "interface". Maybe "called on an object that is not an instance of X"?

[ExE-Boss]

I went with: “'bar' called on an object that is not a valid instance of Foo.”, because Object.create(Foo.prototype) is an instance of Foo, but the WebIDL brand checks will fail, so it’s not a valid instance.

Same applies to ESNext classes with private fields.