cam_operation_add should handle exception if there is no object in scene

[eastshores]

If the user attempts to add an operation with no objects in the scene an exception is thrown. This should be handled to either inform the user that they must add an object or possibly prompt the user to ask if they would like to add a default object - this could allow them to choose from one of several default object types (mesh, curve, etc)

[eastshores]

If no object is in the scene active_object will be none. Rather than raise an exception, this is handled through a self report error asking the user to add an object for the operation.