ACTION-2056 - Summarise compute-exception problem, with an eye to finding a solution

All,

As a reminder, this is about figuring out whether we can remove the last
fatal (non-cancelable) exception, namely compute-exception.

The main question, raised in this editorial note [1], is: what to do if a
problem is encountered during a `recalculate`? There are two problems
raised:

1. an expression violating any Binding Expression Constraint
2. a circular dependency

I am not entirely sure what #1 is, because "Binding Expression Constraint "
is not defined. But we can assume maybe "an issue processing the
expression".

Either way, there can be issues while processing the dependencies during
`recalculate`.

We could try to specify something complicated, but I don't think we have
all the necessary implementor feedback for that. One relatively simple
solution would consist in the following:

- dispatch a new cancelable `compute-error` event
- interrupt the `recalculate` process and clear the `recalculate ` flag

If a form author cancels `compute-error`, which like the other `-error`
events would ,by default, halt XForms processing altogether, it means the
form author hopefully knows what they are doing, and probably just intend
to show a message or same some data.

So I would suggest going in that direction.

-Erik

[1]
https://www.w3.org/community/xformsusers/wiki/XForms_2.0#The_xforms-compute-exception_Event