Do not halt parsing when unknown creatives appear

[pietermees]

See 5eed22f#commitcomment-20401909

Currently when an unknown creative type appears (which is not standards-compliant) the parser fails. It would be valuable if the parsing would continue and just report that one of the nodes failed to parse.

In a live environment, it can happen that non-standard documents show up and we'd like to deal with those.

[timdp]

As suggested in the line notes, an UnknownCreative subclass would work. However, I'm inclined to make it optional behavior. We could, for example, introduce a {strict: true} or {loose: true} option to the parser that (dis)allows this scenario.

[pietermees]

I'm going to try and get such a VAST document. I'm fine with an option.
In general, when the VAST is not coming from us we have no way of
guaranteeing it's standards compliant, and we should gracefully handle any
"noise" without generating "noise" ourselves per FCC guidelines :)