Fix error message for incorrect subtype in encoder.write()

[awann-4ai]

Fixes the error message that is shown when parsing a NbsWritePacket type with a subtype field that is the wrong type or explicitly set as undefined.

It would previously print the error message "invalid type for argument `packet`: A number was expected". These changes make it now print "invalid type for argument `packet`: expected `subtype` to be a number".

[JosephusPaye]

If we're making the subtype required, change the type in NbsWritePacket to reflect that too.

But I think it would be better to keep it optional from the outside and default it to zero before passing it to C++.

Edit: looks like your changes do make it behave as described above. We may just need a test with subtype excluded and subtype set to undefined - this could be done with extra packets in the roundtrip test.