Handle uninstalled custom node

This submission is to fix defect MAGN-1242 Uninstalled custom node loads differently depending on outPort used / prevents Dynamo run.

The previous workflow is if cannot locate the corresponding .dyf for custom node, Dynamo will create an empty CustomNodeDefinition for that node and create a proxy custom node for the first custom node instance. As Dynamo now can locate definition for the remaining custom node instances, these instances appear to be "normal".

The fix is to mark all custom node instances as Error by default and add a propery IsProxy to CustomNodeDefinition to mark it is a proxy definition . Once custom node is properly loaded into Dynamo, the new definition will be created and all custom node instances will resync to the new one.