Better error message for ((a, b) => c): (Tuple2[A, B] => C)

Stopping short of just DWIM, as one gets with pattern matching
anonymous function, offer a diagnostic to guide the user towards
the supported way of destructuring a `Tuple` in the sole argument
of a `Function1`.
Better error message for (((a, b)) => c)

Explain that this syntax isn't valid and point out alternatives.