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/tests.rs

@@ -103,10 +103,7 @@ fn conn_reset_after_write() {
         Ok(Async::Ready(()))
     }).map_err(|e: ::std::io::Error| panic!("srv2 poll_fn error: {}", e));
     let err = rt.block_on(res2.join(srv2)).expect_err("res2");
-    match err.kind() {
-        &::error::Kind::Incomplete => (),
-        other => panic!("expected Incomplete, found {:?}", other)
-    }
+    assert!(err.is_incomplete_message(), "{:?}", err);
 }
 
 #[test]