Add rustc_on_unimplemented message to std::ops::Try

src/libcore/ops/try.rs

@@ -15,6 +15,8 @@
 /// extracting those success or failure values from an existing instance and
 /// creating a new instance from a success or failure value.
 #[unstable(feature = "try_trait", issue = "42327")]
+#[rustc_on_unimplemented = "the `?` operator can only be used in a function that returns `Result` \
+                            (or another type that implements `std::ops::Try`)"]
 pub trait Try {
     /// The type of this value when viewed as successful.
     #[unstable(feature = "try_trait", issue = "42327")]