FileNotFoundError --> IOError exception bug fix

Former-commit-id: c1d75ff4c9

opendm/gsd.py

@@ -72,7 +72,7 @@ def opensfm_reconstruction_average_gsd(reconstruction_json):
         a GSD estimate cannot be compute
     """
     if not os.path.isfile(reconstruction_json):
-        raise FileNotFoundError(reconstruction_json + " does not exist.")
+        raise IOError(reconstruction_json + " does not exist.")
 
     with open(reconstruction_json) as f:
         data = json.load(f)