Note user-facing types of coercion failure

[compiler-errors]

When coercing, for example, Box<A> into Box<dyn B>, make sure that any failure notes mention those specific types, rather than mentioning inner types, like "the cast from A to dyn B".

I expect end-users are often confused when we skip layers of types and only mention the "innermost" part of a coercion, especially when other notes point at HIR, e.g. #111406.