Make Entity::isNew() only support true/false

Having an indeterminate state complicates life. Having only true/false
makes interacting with the ORM a bit simpler. Table::save() will still
determine if an entity is truely new when saving and the primary key is
present. Delete operations assume you know what you are doing, and don't
try to help as destroying data is generally more dangerous/difficult to
recover vs. having too much data.

Refs #4050
Clarify error message.
Merge pull request #4080 from cakephp/issue-4050-isNew-boolean

Make Entity::isNew() only support true/false