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/proto/h2/mod.rs

@@ -126,7 +126,7 @@ where
                         match try_ready!(self.body_tx.poll_capacity().map_err(::Error::new_body_write)) {
                             Some(0) => {}
                             Some(_) => break,
-                            None => return Err(::Error::new_canceled(None::<::Error>)),
+                            None => return Err(::Error::new_canceled()),
                         }
                     }
                 } else {