[Ur] Catching errors

[Edward Z. Yang]

Is there a way to catch an 'error <xml>asdf</xml>'?  For example
the json module uses error to indicate parse failure, and instead
I'd like to catch it and try something else.

The onError function seems like a partial mechanism for this, but
I don't understand how scoping on onError works; if I register
something as onError, is it permanently called on error? Is it
displaced by the next onError I call?

Edward