Crash early before downloading images, etc, when no data is extracted at all

Closes #220 (closed)

dan/datasets/extract/arkindex.py

@@ -481,6 +481,11 @@ class ArkindexExtractor:
                     pbar.update()
                     pbar.refresh()
 
+        if not self.data:
+            raise Exception(
+                "No data was extracted using the provided export database and parameters."
+            )
+
         self.download_images()
         self.format_lm_files()
         self.export()