refactor(error): improve organization of Error kinds

- Placed all cases of "unexpected bytes" errors into the
  `UnexpectedMessage` variant.
- Placed all cases of "unexpected EOF" errors into the
  `IncompleteMessage` variant. Description is now generic about
  "connection closed before message completed", instead of mentioning
  "request" or "response.
- Added `Error::is_incomplete_message()` accessor to help checking for
  unexpected closures.
- Renamed some variants to be clearer when viewing the `Debug` format.
- Collected all "user" errors into an internal `User` enum, to prevent
  forgetting to update the `is_user()` method.

src/upgrade.rs

@@ -209,7 +209,7 @@ impl Future for OnUpgrade {
                 Ok(Async::Ready(Ok(upgraded))) => Ok(Async::Ready(upgraded)),
                 Ok(Async::Ready(Err(err))) => Err(err),
                 Err(_oneshot_canceled) => Err(
-                    ::Error::new_canceled(Some(UpgradeExpected(())))
+                    ::Error::new_canceled().with(UpgradeExpected(()))
                 ),
             },
             None => Err(::Error::new_user_no_upgrade()),