Update project documentation to account for typed errors

[thomashoneyman]

What does this pull request do?

Updates the README to account for the new typed errors implementation in #73.

Other Notes:

I also made a pair of other small updates.

1. I added a parseJson function that parses String -> Either JsonDecodeError Json. This is mostly for convenience -- it's possible to just use jsonParser from argonaut-core and then lmap (TypeMismatch "JSON"), but I think this is a bit much to expect a regular user to have to bikeshed about in their codebase. Providing a function that gets Json from a String and which is already compatible with the decoders seems like the way to go. It's called parseJson to avoid a name conflict with jsonParser.
2. I renamed the Errors module to Error, as it only defines a single error type and exports it.