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/client/dispatch.rs

@@ -167,7 +167,7 @@ struct Envelope<T, U>(Option<(T, Callback<T, U>)>);
 impl<T, U> Drop for Envelope<T, U> {
     fn drop(&mut self) {
         if let Some((val, cb)) = self.0.take() {
-            let _ = cb.send(Err((::Error::new_canceled(None::<::Error>), Some(val))));
+            let _ = cb.send(Err((::Error::new_canceled().with("connection closed"), Some(val))));
         }
     }
 }