Allow DOMException subclasses to be used as exceptions

Also be clearer about how exception names are meant to be used, at least for now (pending discussion in #1219).

See discussion in #1168 (comment). Closes #1223.

This also includes some updates to all exception creation and throwing, such as reflecting how messages are actually passed along in practice, or using Web IDL infrastructure to create DOMException instances instead of Construct()ing them.

Author: domenic