Is your feature request related to a problem? Please describe.
A problem that we encounter when using cast_stored and dump_to_native on embeds is that it can be very difficult to determine where an error occurred, because there are no error messages for those callbacks. So you could have a 6-nested embedded resource and something deep in there could fail to cast to/from the database and all you would get is :error.

Describe the solution you'd like
Support error messages in cast_stored and dump_to_native, i.e {:error, "Error message"}. This would be used in embeds to return which key specifically failed the cast.