message: [web] verbose invalid path exception
  This commit outputs more exception information when the path is invalid.
diff: @@ -205,7 +205,7 @@ void WebServer::DefaultHttpResponse(void)
  } catch (const std::exception &e)
  {
  - std::string content = "Could not open path";
  + std::string content = "Could not open path `" + request->path + "`: " + e.what();
  *response << OT_RESPONSE_FAILURE_STATUS << OT_RESPONSE_HEADER_LENGTH << content.length()
  << OT_RESPONSE_PLACEHOLD << content;
  }
  