descriptor: PoolError::InvalidJsonName covers two different failures, and MalformedMapEntry::message holds a field name #641

Three PoolError shapes in buffa-descriptor/src/pool.rs are worth settling before the next breaking release, since variant and field names are public API.

• InvalidJsonName { field, name } reports two unrelated rules: a JSON name that contains NUL, and a non-extension JSON name of the form [...]. Display re-derives which one from name.contains('\0'), and a caller that matches the variant has to do the same. The variant is unreleased, so splitting it (or adding a reason field) costs nothing now.
• MalformedMapEntry { message } carries the full name of the map field that uses the entry, as its docs say; the entry message is that field's type. Every other variant uses field for a field name and message for a message name. This one has shipped since 0.7.0, so the rename to field is a breaking change.
• MissingTypeName { field } is also returned for an extension with no extendee and a method with no input_type or output_type (resolve_message_type_name). Its message then reads "field pkg.Svc.Method has no type_name", which names neither the missing attribute nor the right kind of declaration.