Uncaught NameError when trying to free objects that have not been created

[askorikov]

If execution fails before the corresponding Astra objects were created, an uncaught NameError is raised here and other functions in the module:

flexTOMO/flextomo/projector.py

Lines 686 to 688 in 126b191

	astra.algorithm.delete(projector_id)
	astra.data3d.delete(sin_id)
	astra.data3d.delete(vol_id)

I can open a pool request if this isn't intended behavior.