Revert deletions of client_error and friends.

src/request.rs

@@ -121,8 +121,8 @@ impl Request {
         let response =
             unit::connect(unit, true, 0, reader, false).map_err(|e| e.url(url.clone()))?;
 
-        if self.error_on_non_2xx && response.status() >= 400 {
-            Err(Error::Status(response.status(), response))
+        if response.error() && self.error_on_non_2xx {
+            Err(ErrorKind::HTTP.new().url(url.clone()).response(response))
         } else {
             Ok(response)
         }