Define new error "Unknown Markup"

[aphillips]

From a discussion about the ABNF with @eemeli I noticed that we don't have an error for when markup is not recognized. Proposing a fix.

[mihnita]

I think that markup is still not agreed on.
So adding some error for it only adds stuff to cleanup after.

[mihnita]

Markup open issues:

• Markup: do we need a different sigil for display element names? #241
• Do markup elements map to registered functions? #262
• Markup: how do ensure non-HTML display elements are first class? #238

And to show that it was not agreed on, see PR #283 in Jun 21, 2022:

I will approve it to move things forward and not have these many open PRs.
It does not mean I agree with this syntax, so it should not be hold against me 1 month from now ("but you approved X")
There are still at least 2 open issues arguing about how to best handle "markup elements"

[eemeli]

This looks good, and well in line with the Unknown Function error.

Some nitpicks inline.

[aphillips]

close in telecon 2023-05-22