Lack of dx, dy maps affects processing, so be strict about loading

them. If they are expected but not loaded, then fail.

Thanks to @ndevenish for the suggestion.

src/dxtbx/model/experiment_list.py

@@ -334,6 +334,14 @@ def _imageset_from_imageset_data(self, imageset_data, models):
         dx_filename, dx = self._load_pickle_path(imageset_data, "dx")
         dy_filename, dy = self._load_pickle_path(imageset_data, "dy")
 
+        # If dx, dy maps are expected then they must be loaded even when
+        # self._check_format == False, because they affect the operation of
+        # programs (dials.index, dials.refine) that do not need the image data.
+        if (dx_filename or dy_filename) and not all((dx, dx)):
+            raise RuntimeError(
+                f"dx ({dx_filename}) and dy ({dy_filename}) maps are expected"
+            )
+
         if imageset_data["__id__"] == "ImageSet":
             imageset = self._make_stills(imageset_data, format_kwargs=format_kwargs)
         elif imageset_data["__id__"] == "ImageGrid":