Throw an exception when an undefined method is called on one of our DB_DataObjects...

Throw an exception when an undefined method is called on one of our DB_DataObjects, instead of failing silently.
The magic __call() method is used to implement a getter and setter interface, and simply didn't bother to throw an error for things it didn't recognize.

This may expose a number of existing errors where mistyped method names are called and we're not noticing that they're failing.