Noted inaccuracies of the is_error function for Response.

src/proto/response.rs

@@ -408,6 +408,10 @@ make_response! {
 
 impl Response {
     /// Determines whether or not this response is an error response.
+    ///
+    /// This error consideration is according to RFC2812, but is rather simplistic. It considers all
+    /// response codes above 400 to be errors, which misclassifies some extensions (e.g. from IRCv3)
+    /// that add responses and errors both in the same range (typically 700s or 900s).
     pub fn is_error(&self) -> bool {
         *self as u16 >= 400
     }