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

Closes #220 (closed)

dan/datasets/extract/arkindex.py

@@ -483,7 +483,7 @@ class ArkindexExtractor:
 
         if not self.data:
             raise Exception(
-                "No data was extracted using the provided export database and parameters"
+                "No data was extracted using the provided export database and parameters."
             )
 
         self.download_images()