change Message.code type to uint32

proto/caosdb/entity/v1alpha1/main.proto

@@ -29,7 +29,7 @@ package caosdb.entity.v1alpha1;
 // errors.
 message Message {
   // The code is a machine-readable identifier.
-  string code = 1;
+  uint32 code = 1;
   // A short description of the message which is indented for human readers.
   string description = 2;
 }