Misleading error message on use of FnOnce<(T, U), Output=V>

[This stack overflow question](http://stackoverflow.com/questions/31353960/how-to-use-parentheses-for-generics/31354984?noredirect=1#comment50696461_31354984) shows that if one tries to declare a `FnOnce<(T, U), Output=V>` (which should be a `FnOnce(T, U) -> V`), the error message is quite misleading.
