Blender Git Loki

NLA Pushdown Operator: Active animdata option + More error handling

* If channel_index = -1, the pushdown operator will try to use the active AnimData
block. To see which one this will be, check the properties (NKEY) region in NLA Editor.
* Added more detailed error handling for the various cases where things are not
in a valid state for pushing down an action. This is notably to stop scripters
from putting the NLA into an inconsistent state.