Provide a witness pattern for non-exhaustive patterns

Fixed #4321

src/test/compile-fail/refutable-pattern-errors.rs

@@ -10,9 +10,9 @@
 
 
 fn func((1, (Some(1), 2..3)): (int, (Option<int>, int))) { }
-//~^ ERROR refutable pattern in function argument
+//~^ ERROR refutable pattern in function argument: (_, _) not covered
 
 fn main() {
     let (1, (Some(1), 2..3)) = (1, (None, 2));
-    //~^ ERROR refutable pattern in local binding
+    //~^ ERROR refutable pattern in local binding: (_, _) not covered
 }