Wrapped iterator's "throw"

[mhofman]

Since (async) iterator wrapper object unconditionally have a "throw", they need to handle the case of the wrapped iterator lacking a "throw" method. yield * goes to some length to fallback to closing the iterator before throwing a type error. The existing sync to async iterator wrapper is surprisingly not following the same behavior, but consensus was reached at the last plenary on my PR to fix it: tc39/ecma262#2600

[bakkot]

As of #194 iterator wrapper objects no longer have throw.