Use a Spark option schema to define error fields.

[jimsynz]

Gives us the following advantages:

• Autocomplete for options to MyError.exception/1.
• Can automatically define t:MyError.t
• Can automatically generate documentation.

[zachdaniel]

I was thinking there might be a chicken and the egg type problem between spark and splode, but we'll just have to choose one really, either spark can produce splode errors, or splode can leverage spark features 😆