ZeroLayer required - catch this

[demisjohn]

If you disable the line:

ZeroLyr = MyJob.Layer() # Empty Layer with default values

Then the conversion with ASML JobCreator on the target tool will fail.
The ASML converter fails with:

ERROR: JD-0001
Reticle Data - Layer ID (1) Image ID (P1.2D200_00)
Image cannot be used, because it does not exist in this layer.

And then the JDJC converter crashes with an Exception 11

Currently, If the ZeroLayer is not defined, then the python script adds images to Layer "0".

Instead, should catch whether there is no Zero layer, and instead start adding Layers at # 1 (not 0).