add docs for IncompleteMessage and other Error kinds

[jmhodges-signal]

It'd be cool to get the docs on IncompleteMessage and similar Kinds on docs.rs. Currently, they're hidden and google searching finds you to the source code first.

They seemed to have existed in ancient history, but no longer: https://docs.rs/hyper/0.11.22/hyper/error/enum.Error.html