directly report if no rrule/frule is defined #81
Description
Right now we hit the fallback definition of rrule/frulewhich is to just returnnothing`.
which then gives an error complaining about iteration failing.
We should instead detect that an error ourselves with somehting more clear.
Possibly throwing a MethodError which would show what is closest.
(though it would also show the Any method that does match but maybe that is ok)