Tweak E0038 output to be more pedagogic.

[estebank]

Given:

pub trait Item {
    fn get_category()->String; // error
    fn do_thing_inner(&self);
    fn get_inner_val(&self) -> u32;
}

impl dyn Item {
    pub fn do_thing(&self) {
        self.do_thing_inner();
        //etc.
    }
    pub fn get_val(&self) -> u32 {
        self.get_inner_val() + 1
    }
}

We currently emit

error[E0038]: the trait `Item` cannot be made into an object
 --> src/lib.rs:7:6
  |
1 | pub trait Item {
  |           ---- this trait cannot be made into an object...
2 |     fn get_category()->String; // error
  |        ------------ ...because associated function `get_category` has no `self` parameter
...
7 | impl dyn Item {
  |      ^^^^^^^^ the trait `Item` cannot be made into an object
  |
help: consider turning `get_category` into a method by giving it a `&self` argument or constraining it so it does not apply to trait objects
  |
2 |     fn get_category()->String where Self: Sized; // error
  |                               ^^^^^^^^^^^^^^^^^

and two other similar errors.

Ideally, we would emit only a single error for this case, and produce output closer to

error[E0038]: the trait `Item` cannot be made into an object
 --> src/lib.rs:7:6
  |
7 | impl dyn Item {
  |      ^^^^^^^^ the trait `Item` cannot be made into an object
  |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit https://doc.rust-lang.org/reference/items/traits.html#object-safety
  |
1 | pub trait Item {
  |           ---- this trait cannot be made into an object...
2 |     fn get_category()->String; // error
  |        ^^^^^^^^^^^^ ...because associated function `get_category` has no `self` parameter
  |
help: consider turning `get_category` into a method by giving it a `&self` argument
  |
2 |     fn get_category(&self)->String; // error
  |                     ^^^^^
help: alternatively, consider constraining it so it does not apply to trait objects
  |
2 |     fn get_category()->String where Self: Sized; // error
  |                               ^^^^^^^^^^^^^^^^^

Taken from https://users.rust-lang.org/t/why-cant-i-impl-dyn-a-trait-with-static-methods-and-whats-the-correct-way-to-achieve-this/49637