suggest adding a where-clause when that can help

suggest adding a where-clause when there is an unmet trait-bound that
can be satisfied if some type can implement it.

src/test/compile-fail/no_send-struct.rs

@@ -23,5 +23,5 @@ fn bar<T: Send>(_: T) {}
 fn main() {
     let x = Foo { a: 5 };
     bar(x);
-    //~^ ERROR the trait `std::marker::Send` is not implemented
+    //~^ ERROR `Foo : std::marker::Send` is not satisfied
 }