when making the key the use attribute (conceptually) has to be evaluated
on every node that matches the match attribute, so you know which nodes
are indexed by which key values. If evaluating teh use attribute can
generate an error then you shoud lexpect an error when teh key is made,
which in practice is probably the first time you use the key() function.
the arggument of the function are not really relevant as once the system
decides it needs to build the key it needs to index the whole document.

One can argue other way: key() function should never return a node
for which evaluation of use resulted in an error.

Since it's an extension function ...