proxy: add HTTPErrorHandler that allows for custom handling error han…

…dling

The idea is taken from the Stripe fork [1] but instead of context it's set globally for proxy.
It gives much flexibility to error handling.

[1] https://github.com/stripe/goproxy/blob/3f1dfba6d1a4747c78fee7069e2f28fd6b703917/ctx.go#L21

https.go

@@ -324,6 +324,11 @@ func (proxy *ProxyHttpServer) handleHttps(w http.ResponseWriter, r *http.Request
 }
 
 func httpError(w io.WriteCloser, ctx *ProxyCtx, err error) {
+	if ctx.Proxy.HTTPErrorHandler != nil {
+		ctx.Proxy.HTTPErrorHandler(w, ctx, err)
+		return
+	}
+
 	if _, err := io.WriteString(w, "HTTP/1.1 502 Bad Gateway\r\n\r\n"); err != nil {
 		ctx.Warnf("Error responding to client: %s", err)
 	}

proxy.go

@@ -25,6 +25,8 @@ type ProxyHttpServer struct {
 	respHandlers    []RespHandler
 	httpsHandlers   []HttpsHandler
 	Tr              *http.Transport
+	// Will be invoked to return a custom response to clients when goproxy fails to connect to a proxy target
+	HTTPErrorHandler func(io.WriteCloser, *ProxyCtx, error)
 	// ConnectDial will be used to create TCP connections for CONNECT requests
 	// if nil Tr.Dial will be used
 	ConnectDial        func(network string, addr string) (net.Conn, error)