Gateway to backend API: add default error code

Added to avoid any issue in future when adding new error codes.
It will not solve issue with old backends (using protobuf without
this line) but it will make upgrade easier after that.

Author: GwendalRaoul

gateway_to_backend/protocol_buffers_files/error.proto

@@ -4,6 +4,14 @@ package wirepas.proto.gateway_api;
 
 enum ErrorCode {
     option allow_alias = true;
+    // First enum code is the one returned in case error code
+    // is unknown on the other side (new added values in future)
+    // To avoid having OK as default code, better to use an explicit UNKNOWN_ERROR_CODE.
+    // It will become the default only for the one using a protobuf file that has this
+    // error code defined.
+    // This unknown error code should never be explicitly set from a gateway but used
+    // only by parser when the error code set is unknown
+    UNKNOWN_ERROR_CODE = -1;
     OK = 0;
     INTERNAL_ERROR = 1;
     INVALID_SINK_ID = 2;