Unify looking up attribute name in Xeroizer models

[nnc]

In #419 @stellard made the change to raise a more helpful error when you get the attribute name wrong, which is very helpful btw, but it was limited only to the .build method, while #attributes= still suffers the same issue as .build did before.

This is mostly a reminder to self to unify these two lookups into a common method, so they both raise the more helpful error.
But if anyone else feels like it, please go ahead and submit a pull request, as I'm not at all sure when I'll be able to get around to it.