Make the existential type errors a little bit more helpful

src/test/ui/existential_types/different_defining_uses.stderr

@@ -4,7 +4,7 @@ error: concrete type differs from previous defining existential type use
 LL | / fn bar() -> Foo { //~ ERROR concrete type differs from previous
 LL | |     42i32
 LL | | }
-   | |_^
+   | |_^ expected `&'static str`, got `i32`
    |
 note: previous use here
   --> $DIR/different_defining_uses.rs:8:1

src/test/ui/existential_types/different_defining_uses_never_type.stderr

@@ -4,7 +4,7 @@ error: concrete type differs from previous defining existential type use
 LL | / fn bar() -> Foo { //~ ERROR concrete type differs from previous
 LL | |     panic!()
 LL | | }
-   | |_^
+   | |_^ expected `&'static str`, got `()`
    |
 note: previous use here
   --> $DIR/different_defining_uses_never_type.rs:8:1
@@ -20,7 +20,7 @@ error: concrete type differs from previous defining existential type use
 LL | / fn boo() -> Foo { //~ ERROR concrete type differs from previous
 LL | |     loop {}
 LL | | }
-   | |_^
+   | |_^ expected `&'static str`, got `()`
    |
 note: previous use here
   --> $DIR/different_defining_uses_never_type.rs:8:1

src/test/ui/existential_types/generic_different_defining_uses.stderr

@@ -4,7 +4,7 @@ error: concrete type differs from previous defining existential type use
 LL | / fn my_iter2<T>(t: T) -> MyIter<T> { //~ ERROR concrete type differs from previous
 LL | |     Some(t).into_iter()
 LL | | }
-   | |_^
+   | |_^ expected `std::iter::Once<T>`, got `std::option::IntoIter<T>`
    |
 note: previous use here
   --> $DIR/generic_different_defining_uses.rs:7:1

src/test/ui/existential_types/generic_duplicate_param_use3.stderr

@@ -14,7 +14,7 @@ LL | / fn three<T, U: Debug>(_: T, u: U) -> Two<T, U> {
 LL | | //~^ concrete type's generic parameters differ from previous defining use
 LL | |     u
 LL | | }
-   | |_^
+   | |_^ expected [`T`], got [`U`]
    |
 note: previous use here
   --> $DIR/generic_duplicate_param_use3.rs:15:1

src/test/ui/existential_types/generic_duplicate_param_use5.stderr

@@ -5,7 +5,7 @@ LL | / fn three<T: Debug, U: Debug>(t: T, u: U) -> Two<T, U> {
 LL | | //~^ concrete type differs from previous
 LL | |     (u, t)
 LL | | }
-   | |_^
+   | |_^ expected `(T, U)`, got `(U, T)`
    |
 note: previous use here
   --> $DIR/generic_duplicate_param_use5.rs:10:1

src/test/ui/existential_types/generic_duplicate_param_use6.stderr

@@ -5,7 +5,7 @@ LL | / fn three<T: Debug, U: Debug>(t: T, u: U) -> Two<T, U> {
 LL | | //~^ concrete type differs from previous
 LL | |     (u, t)
 LL | | }
-   | |_^
+   | |_^ expected `(T, T)`, got `(U, T)`
    |
 note: previous use here
   --> $DIR/generic_duplicate_param_use6.rs:10:1