Serialize error class and message separately

[tfausak]

Fixes #42. Fixes #43.

This changes the internal representation of failures to be easier to serialize and deserialize. It also makes .from_json able to handle failure by returning a failure describing the deserialization problem. This will allow it to handle changes to the serialization format in the future.

@justinsteffy: can you review this?

[justinsteffy]

Yeah LGTM. I'd say we will probably want a stack trace, but there is the obvious tradeoff in redis space.

[jpowell]

:+1: