Patch up broken error messages

Author: nikomatsakis

src/libcore/ops.rs

@@ -638,7 +638,7 @@ shr_impl!(uint u8 u16 u32 u64 int i8 i16 i32 i64)
  * ```
  */
 #[lang="index"]
-pub trait Index<Index, Result> {
+pub trait Index<Index, Sized? Result> {
     /// The method for the indexing (`Foo[Bar]`) operation
     fn index<'a>(&'a self, index: &Index) -> &'a Result;
 }

src/test/compile-fail/issue-17636.rs

@@ -15,5 +15,5 @@ pub fn build_archive<'a, I: MyItem<&'a (|&uint|:'a)>>(files: I) {}
 
 fn main() {
     build_archive(&(|_| { }));
-//~^ ERROR unable to infer enough type information to locate the impl of the trait `MyItem<&|&uint|
+//~^ ERROR not implemented
 }

src/test/compile-fail/issue-2149.rs

@@ -18,6 +18,7 @@ impl<A> vec_monad<A> for Vec<A> {
         let mut r = fail!();
         for elt in self.iter() { r = r + f(*elt); }
         //~^ ERROR the type of this value must be known
+        //~^^ ERROR not implemented
    }
 }
 fn main() {