Incorrect flow elements

[borodust]

If, for example, function used in a place where a flow element must be, but doesn't represent a correct flow callback, unobviuos things happen. Condition raised in runtime won't provide much useful information to find a culpirt in this case.

(>> (function-returning-flow)                   ; ok
    (function-returning-neither-flow-nor-list)) ; hard-to-locate error

That's a programming error, but hard to catch atm. Some checks need to be added.