Rollup merge of #21658 - Manishearth:index_on_unimplemented, r=Gankro

Helps issues like [these](http://www.reddit.com/r/rust/comments/2tpefm/unable_to_access_array_elements/)

r? @Gankro

rollup-worthy

src/libcore/ops.rs

@@ -893,6 +893,7 @@ shr_impl_all! { u8 u16 u32 u64 usize i8 i16 i32 i64 isize }
 /// }
 /// ```
 #[lang="index"]
+#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Index}`"]
 #[stable]
 pub trait Index<Index: ?Sized> {
     type Output: ?Sized;
@@ -931,6 +932,7 @@ pub trait Index<Index: ?Sized> {
 /// }
 /// ```
 #[lang="index_mut"]
+#[rustc_on_unimplemented = "the type `{Self}` cannot be mutably indexed by `{Index}`"]
 #[stable]
 pub trait IndexMut<Index: ?Sized> {
     type Output: ?Sized;