Set decorationState.[[Finished]] to true when decorator throws

[nicolo-ribaudo]

Decorators have a "finished" state that is set when the decorator call is done, to prevent addInitializer to be called after the decorator is done.

The current proposed spec sets it to true only if the decorator call does not throw:

class A {
  @((_, ctx) => {
    setTimeout(() => /* throws */ ctx.addInitializer(() => {}), 1000);
  })
  method() {}
}

try {
  class A {
    @((_, ctx) => {
      setTimeout(() => /* throws */ ctx.addInitializer(() => {}), 1000);
      return 3;
    })
    method() {}
  }
} finally {} // Decorating the class throws, because the decorator returns an invalid value

try {
  class A {
    @((_, ctx) => {
      setTimeout(() => /* does not throw */ ctx.addInitializer(() => {}), 1000);
      throw {}
    })
    method() {}
  }
} finally {} // Decorating the class throws, because the decorator throws

This PR updates the third case to also throw.

Fixes tc39/proposal-decorators#500.

[nicolo-ribaudo]

I was too quick to push -- I updated also the other place where we set [[Finished]] to true.