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/mod.rs

@@ -498,7 +498,7 @@ where C: Connect + Sync + 'static,
             let connecting = match pool.connecting(&pool_key, ver) {
                 Some(lock) => lock,
                 None => {
-                    let canceled = ::Error::new_canceled(Some("HTTP/2 connection in progress"));
+                    let canceled = ::Error::new_canceled().with("HTTP/2 connection in progress");
                     return Either::B(future::err(canceled));
                 }
             };
@@ -517,7 +517,7 @@ where C: Connect + Sync + 'static,
                             None => {
                                 // Another connection has already upgraded,
                                 // the pool checkout should finish up for us.
-                                let canceled = ::Error::new_canceled(Some("ALPN upgraded to HTTP/2"));
+                                let canceled = ::Error::new_canceled().with("ALPN upgraded to HTTP/2");
                                 return Either::B(future::err(canceled));
                             }
                         }