Post condition check

Former-commit-id: 954ba33db0

opendm/location.py

@@ -33,6 +33,9 @@ def extract_utm_coords(photos, images_path, output_coords_file):
         
         coords.append(coord)
 
+    if utm_zone is None:
+        raise Exception("No images seem to have GPS information")
+        
     # Calculate average
     dx = 0.0
     dy = 0.0