Closure error messages should explain how the return type got inferred

[Aaron1011]

The following code:

fn main() {
    || {
        if false {
            return 1;
        }
    };
}

produces the following error message:

error[E0308]: mismatched types
 --> src/main.rs:3:9
  |
3 | /         if false {
4 | |             return 1;
5 | |         }
  | |_________^ expected integer, found `()`

The message doesn't explain why an integer was expected here. If the user doesn't explicitly annotate the return type of the closure, we should point to the expression that caused the return type to get inferred to begin with.