This small commit stabilizes the `Error` trait as-is, except that `Send`
and `Debug` are added as constraints. The `Send` constraint is because
most uses of `Error` will be for trait objects, and by default we would
like these objects to be transferrable between threads. The `Debug`
constraint is to ensure that e.g. `Box<Error>` is `Debug`, and because
types that implement `Display` should certainly implement `Debug` in any case.
In the near future we expect to add `Any`-like downcasting features to
`Error`, but this is waiting on some additional
mechanisms (`Reflect`). It will be added before 1.0 via default methods.
[breaking-change]
r? @alexcrichton
Closes#21790
b97e1cc28e