Explain why formula can't be translated to TPTP-FOF

[fcbr]

It would be great for debugging purposes if we could output why a particular formula isn't eligible to be translated to TPTP/FOF. Basically it would entail changing this code https://github.com/own-pt/cl-krr/blob/master/tptp.lisp#L81-L96 to return a reason instead of just true and false.

[arademaker]

two errors in a recent implementation prevent the generation of fof code without warning:

1. function not defined (e.g. InList vs. inList)

2. use of terms in predicate position (e.g. (=> ListFn(...) ...)