Improve span of missing Display impl error

[dtolnay]

Before:

error[E0277]: `MyError` doesn't implement `std::fmt::Display`
 --> $DIR/missing-display.rs:3:10
  |
3 | #[derive(Error, Debug)]
  |          ^^^^^ `MyError` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `MyError`

After:

error[E0277]: `MyError` doesn't implement `std::fmt::Display`
 --> $DIR/missing-display.rs:4:1
  |
4 | pub enum MyError {
  | ^^^^^^^^^^^^^^^^ `MyError` cannot be formatted with the default formatter
  |
  = help: the trait `std::fmt::Display` is not implemented for `MyError`

Closes #75.