improve error message on attempt to impl on type alias

[binarycrusader]

As seen previously in #9767, messaging when attempting to impl a trait involving a type alias could be improved (using current nightly):

error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
   --> manifest/mod.rs:169:1
    |
169 |   impl<'a> FromIterator<io::Result<String>> for Result<Manifest<'a>> {
    |  _^ starting here...
170 | |     fn from_iter<I: IntoIterator<Item = io::Result<String>>>(iter: I) -> Result<Self> {
171 | |         let mut m = Manifest::new();
172 | |         Ok(m)
173 | |     }
174 | | }
    | |_^ ...ending here: impl doesn't use types inside crate
    |
    = note: the impl does not reference any types defined in this crate

From line 86 of that file:

pub type Result<Manifest> = ::std::result::Result<Manifest, ManifestError>;

I had expected to see = note: define and implement a trait or new type instead as described in #9767, but did not, so perhaps this is a regression or a subtly different case?

As someone relatively new to rust, it seems like I should be able to impl in this case since it is specialized such that it's only for a Result using my types, but I guess not.

[durka]

Looks like we could just duplicate this line to here?