Improve errors after re rebalance coherence

[ohadravid]

Following #65247, I noticed that some error messages should be updated to reflect the changes of re_rebalance_coherence (also there was a note in the RFC about it).

First, error message E0210 was updated to match the RFC, and I also tried to improve a little the error when the "order" of types is problematic.

For code like this:

#![feature(re_rebalance_coherence)] // Now stable

struct Wrap<T>(T);

impl<T> From<Wrap<T>> for T {
    fn from(x: Wrap<T>) -> T {
        x.0
    }
}

The old error was:

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
 --> src/lib.rs:5:6
  |
5 | impl<T> From<Wrap<T>> for T {
  |      ^ type parameter `T` must be used as the type parameter for some local type
  |
  = note: only traits defined in the current crate can be implemented for a type parameter

and the new error is:

error[E0210]: type parameter `T` must be covered by another type when it appears before the first local type (`Wrap<T>`)
  --> main.rs:66:6
   |
66 | impl<T> From<Wrap<T>> for T {
   |      ^ type parameter `T` must be covered by another type when it appears before the first local type (`Wrap<T>`)
   |
   = note: implementing a foreign trait is only possible if at least one of the types for which is it implemented is local, and no uncovered type parameters appear before that first local type
   = note: in this case, 'before' refers to the following order: `impl<..> ForeignTrait<T1, ..., Tn> for T0`, where `T0` is the first and `Tn` is the last

I tried to point at the uncovered T, but couldn't get something which was reliable (but I'll be happy to try if someone points me in the right direction).

r? @estebank
cc @nikomatsakis

Fixes #65247

[rust-highfive]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[estebank]

@bors r+

[bors]

📌 Commit 2db744c has been approved by estebank