Stabilize verification of message types

[SSheldon]

Verifying argument types for messages was added back with #10. It should remain behind a feature, but before it ships a couple things should be done:

• What type should the error be? String isn't a very rust-y error type.
• Should there be a public API for verifying these types? Or is panicking internally when they're incorrect sufficient?
• The feature should be documented.

[SSheldon]

A MessageError type was introduced in 6a6d714. It's currently just a light wrapper around a String, but it provides a level of abstraction so it can be improved in the future.

[SSheldon]

Added Message::verify_message in e187ba8. I think all that's left here is doing a final pass on the docs.