JSONExtensions

Some minor supporting utilities for Swift JSON parsing.

Error Translation

Adds isJSONError and jsonErrorDescription to Error, which provide a way to test for JSON parsing errors, and a better description of them.

JSONSerialization

Adds a convenience method to decode direct from a String.

Adds convenience decoding methods which don't throw, but instead take an error handler. The error handler is passed the thrown error, but also a pre-formatted description of the error, produced by jsonErrorDescription.