Require "abort algorithms" to not throw?

[domenic]

In https://github.com/whatwg/streams/pull/1132#discussion_r645345792 @ricea asked whether "signal abort" could throw. It appears it could do so if and only if one of the "abort algorithms" added by other specs throws an exception.

Since it seems likely other specs would not anticipate this, can we make it a normative requirement that all abort algorithms are non-throwing?

We could also do something more complicated like catching their exceptions and reporting them, but that seems unnecessary.

[annevk]

It would be nice if we could assert this somehow. We should probably also note that return values are ignored.