feat: error responses return body upon non 101 status code

modified `client::Response` type to contain `String` instead of `()` to achieve this.

src/handshake/client.rs

@@ -26,7 +26,7 @@ use crate::{
 pub type Request = HttpRequest<()>;
 
 /// Client response type.
-pub type Response = HttpResponse<()>;
+pub type Response = HttpResponse<Option<String>>;
 
 /// Client handshake role.
 #[derive(Debug)]
@@ -83,7 +83,7 @@ impl<S: Read + Write> HandshakeRole for ClientHandshake<S> {
                 ProcessingResult::Continue(HandshakeMachine::start_read(stream))
             }
             StageResult::DoneReading { stream, result, tail } => {
-                let result = self.verify_data.verify_response(result)?;
+                let result = self.verify_data.verify_response(result, &tail)?;
                 debug!("Client handshake done.");
                 let websocket =
                     WebSocket::from_partially_read(stream, tail, Role::Client, self.config);
@@ -178,11 +178,12 @@ struct VerifyData {
 }
 
 impl VerifyData {
-    pub fn verify_response(&self, response: Response) -> Result<Response> {
+    pub fn verify_response(&self, mut response: Response, tail: &[u8]) -> Result<Response> {
         // 1. If the status code received from the server is not 101, the
         // client handles the response per HTTP [RFC2616] procedures. (RFC 6455)
         if response.status() != StatusCode::SWITCHING_PROTOCOLS {
-            return Err(Error::Http(response.map(|_| None)));
+            *response.body_mut() = Some(String::from_utf8_lossy(tail).to_string());
+            return Err(Error::Http(response));
         }
 
         let headers = response.headers();
@@ -255,7 +256,7 @@ impl<'h, 'b: 'h> FromHttparse<httparse::Response<'h, 'b>> for Response {
 
         let headers = HeaderMap::from_httparse(raw.headers)?;
 
-        let mut response = Response::new(());
+        let mut response = Response::new(None);
         *response.status_mut() = StatusCode::from_u16(raw.code.expect("Bug: no HTTP status code"))?;
         *response.headers_mut() = headers;
         // TODO: httparse only supports HTTP 0.9/1.0/1.1 but not HTTP 2.0