Reduce verbosity of E0277 for Debug and Display

[estebank]

On missing Debug implementation, the current output is unnecessarily verbose:

error[E0277]: `Group` doesn't implement `Debug`
  --> src/tests/a.rs:19:5
   |
19 |     assert_eq!(result.unwrap(), &group);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Group` cannot be formatted using `{:?}`
   |
   = help: the trait `Debug` is not implemented for `Group`
   = note: add `#[derive(Debug)]` or manually implement `Debug`
   = note: required because of the requirements on the impl of `Debug` for `&Group`
   = note: 1 redundant requirements hidden
   = note: required because of the requirements on the impl of `Debug` for `&&Group`
   = note: required by `std::fmt::Debug::fmt`
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

We should special case this and Display to give less overwhelming output:

error[E0277]: `Group` doesn't implement `Debug`
  --> src/tests/a.rs:19:5
   |
19 |     assert_eq!(result.unwrap(), &group);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `Group` cannot be formatted using `{:?}`
   |
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
help: add `#[derive(Debug)]` to `Group` or manually `impl Debug for Group`
  |
X | #[derive(Debug)] pub struct Group {
  |

Taken from https://blog.frankel.ch/start-rust/1/

[inquisitivecrystal]

I'll see if I can figure this out.

@rustbot claim