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/client.rs

@@ -100,7 +100,7 @@ where
                         Ok(Async::Ready(Some((req, cb)))) => {
                             // check that future hasn't been canceled already
                             if cb.is_canceled() {
-                                trace!("request canceled");
+                                trace!("request callback is canceled");
                                 continue;
                             }
                             let (head, body) = req.into_parts();
@@ -159,11 +159,11 @@ where
 
                         Ok(Async::NotReady) => return Ok(Async::NotReady),
 
-                        Ok(Async::Ready(None)) |
-                        Err(_) => {
+                        Ok(Async::Ready(None)) => {
                             trace!("client::dispatch::Sender dropped");
                             return Ok(Async::Ready(Dispatched::Shutdown));
-                        }
+                        },
+                        Err(never) => match never {},
                     }
                 },
             };

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 {