Begin a design diary.
Make no matching usage a plan old expcetion.

I've made it so that no matching usage message throws a plain old
exception.

Closes #46.
Notes on hiding help exception.
Pass `Error` to `abend` callback.

Instead of passing a litany to the `abend` callback, pass an `Error`
that is ready to rethrow if that is the desired behavior.

Closes #45.
Use a usage message for parse errors.

I'm now using an arguable usage message to express the error messages
raised by the argument parser.

Closes #44.
Merge branch `messages` into `master`.