Altas may not export if one or more layers are missing

[andrewphilipsmith]

How to reproduce

1. Have a MapRecipe within includes multiple layers and an atlas definition.
2. Make just the data for the index layer available but no data for any other layers.
3. Run mapchef.

Expected behaviour

Mapchef should export a pdf for each for the polygons in the index layer, just with the missing data missing from the maps. (This is the same behaviour as would occur if it were not an atlas).

Actual behaviour

The export fails with the following error:

Traceback (most recent call last):
  File "c:\gocd\agent\pipelines\kenya\mapy-arcmap\mapactionpy_arcmap\arcmap_runner.py", line 627, in <module>
    main()
  File "c:\gocd\agent\pipelines\kenya\mapy-arcmap\mapactionpy_arcmap\arcmap_runner.py", line 619, in main
    runner.export()
  File "c:\gocd\agent\pipelines\kenya\mapy-arcmap\mapactionpy_arcmap\arcmap_runner.py", line 269, in export
    export_params = self._do_export(export_params)
  File "c:\gocd\agent\pipelines\kenya\mapy-arcmap\mapactionpy_arcmap\arcmap_runner.py", line 320, in _do_export
    self._export_atlas(self.recipe, arc_mxd, export_dir, core_file_name)
  File "c:\gocd\agent\pipelines\kenya\mapy-arcmap\mapactionpy_arcmap\arcmap_runner.py", line 364, in _export_atlas
    arc_lyr = arcpy.mapping.ListLayers(arc_mxd, None, arc_df)[lyr_index]
IndexError: list index out of range