Clarify failing postcondition of trait method about prusti-dev

[JonasAlaif]

This is still an issue:

use prusti_contracts::*;

trait Foo {
    #[ensures(result > 10)] // Failing postcondition. The verification error is currently reported here.
    fn bar() -> usize;
}

struct A;

impl Foo for A {
    fn bar() -> usize { // Bad implementation. The verification error should be reported here.
        0
    }
}

struct B;

impl Foo for B {
    fn bar() -> usize { // Correct implementation
        100
    }
}

fn main(){}

[Aurel300]

The failing implementation span is now added as a "note" to the error:

error: [Prusti: verification error] postcondition might not hold.
  --> example.rs:4:15
   |
4  |     #[ensures(result > 10)] // Failing postcondition. The verification error is currently reported here.
   |               ^^^^^^^^^^^
   |
note: the error originates here
  --> example.rs:11:5
   |
11 | /     fn bar() -> usize { // Bad implementation. The verification error should be reported here.
12 | |         0
13 | |     }
   | |_____^

Do we want to improve this any further? I think reporting the failing postcondition (or conjunct) as the primary span makes sense.