Revert deletions of client_error and friends.

src/response.rs

@@ -145,6 +145,30 @@ impl Response {
             .collect()
     }
 
+    /// Whether the response status is: 200 <= status <= 299
+    pub fn ok(&self) -> bool {
+        self.status >= 200 && self.status <= 299
+    }
+
+    pub fn redirect(&self) -> bool {
+        self.status >= 300 && self.status <= 399
+    }
+
+    /// Whether the response status is: 400 <= status <= 499
+    pub fn client_error(&self) -> bool {
+        self.status >= 400 && self.status <= 499
+    }
+
+    /// Whether the response status is: 500 <= status <= 599
+    pub fn server_error(&self) -> bool {
+        self.status >= 500 && self.status <= 599
+    }
+
+    /// Whether the response status is: 400 <= status <= 599
+    pub fn error(&self) -> bool {
+        self.client_error() || self.server_error()
+    }
+
     /// The content type part of the "Content-Type" header without
     /// the charset.
     ///