When not finding a method for a type, but finding it for a single other impl, special case the output to emit a single message instead of a list.

```
error[E0599]: no associated item named `C` found for struct `Fail<i32, u32>` in the current scope
  --> $DIR/wrong-projection-self-ty-invalid-bivariant-arg2.rs:15:23
   |
LL | struct Fail<T: Proj<Assoc = U>, U>(T);
   | ---------------------------------- associated item `C` not found for this struct
...
LL |     Fail::<i32, u32>::C
   |                       ^ associated item not found in `Fail<i32, u32>`
   |
   = note: the associated item was found for `Fail<i32, i32>`
```
instead of

```
   = note: the associated item was found for
           - `Fail<i32, i32>`
```