Do not return HPE_USER from where it is prohibited

src/shrpx_http_downstream_connection.cc

@@ -891,8 +891,7 @@ int htp_msg_begincb(llhttp_t *htp) {
   auto downstream = static_cast<Downstream *>(htp->data);
 
   if (downstream->get_response_state() != DownstreamState::INITIAL) {
-    llhttp_set_error_reason(htp, "HTTP message started when it shouldn't");
-    return HPE_USER;
+    return -1;
   }
 
   return 0;

src/shrpx_https_upstream.cc

@@ -559,8 +559,7 @@ int htp_msg_completecb(llhttp_t *htp) {
       // signal_write() to run on_write().
       return HPE_PAUSED;
     }
-    llhttp_set_error_reason(htp, "could not finish request body");
-    return HPE_USER;
+    return -1;
   }
 
   if (handler->get_http2_upgrade_allowed() &&