rmp_serde: TypeMismatch is not very informative

If I try to deserialize and get a TypeMismatch error, it tells me the actual type of the data, but it doesn't tell me the type that was expected. It would be easier to track down this kind of error if both were reported. (It would be even better if rmp_serde could report the location in the data where the error took place, but I'm not sure how feasible that would be.)