Improve Throwable serialization

[BrandonArp]

When serializing throwables, a naive approach is used. Instead, use the same mechanisms that are used to serialize other objects.

[vjkoskela]

Addressed in #42

[vjkoskela]

This was only partially addressed in #42. Although exception data in the exception block is serialized in a way to be consistent with data serialization for additional fields, this does not apply to exceptions in the data block. That is exceptions in the data block do not serialize as Steno exceptions.