One of the section in the JSON I am dealing with is dedicated to errors.  If a given node, call it $.error is set, then $.error.reason contains a text description of the error that occurred.

I can test for the presence of $.error but if I try to use 'custom on fail', set error to $.error.reason that does not get evaluated as the condition does and I just end up with the literal string '$.error.reason' in the error.

The request is to evaluate those expressions in the custom on fail / set Value to / set error to options..