catch KeyError as well as IOError in _load_data

toolkit.py

@@ -409,7 +409,7 @@ class ClassificationProject(object):
             # if those don't exist, we need to load them from ROOT trees first
             self._load_from_hdf5(*self.dataset_names_tree)
 
-        except KeyError:
+        except (KeyError, IOError):
 
             logger.info("Couldn't load all datasets - reading from ROOT trees")