Improve error message for immutable desired state

If you try to change a property of the desired state (which is immutable), you will get the following JavaScript default message:
> Cannot assign to read only property 'arn' of object '#'

We should tell the developer to instantiate a new Resource Model, instead.

Search the repository for the immutable desired-state handling and the current "Cannot assign to read only property" message, then trace how ResourceModel instances are created. Update the resulting error guidance to tell developers to instantiate a new Resource Model and verify that the revised message appears for an attempted property change.

Written by the indexing model from the issue text.