Friendlier error when user puts wrong type

[dgasull]

When there is an error in the class definition, e.g.

class Whatever(DataClayObject):
    """
    ...
    @ClassField some_attr invalid_name
    ...
    """
    ...

Throw a better hint to what is the error. Now, the system complains that cannot split . because it expects a dataClay class signature (i.e.: namespace.class.spec.MyClass). But if there is a typo or something else, it would be nice to include which class triggered the error and on which attribute.

[Writing this issue because CLASS has been stucked on an error and sending the incorrect class file for some days, and tbh, it was not their fault]