[spec] When fspec fails due to return value, `:pred` is not a valid spec in problem

Repro:

Expected: There should be some way to get the spec (i.e. the function itself, not a symbol) that fails for the return value. That way, we can recursively describe why the return value fails the spec.

Note that for a non-fspec failure, the function itself (not the symbol) is included in the explain-data: