[core] Graph: fix Graph.load missing return value

Add missing return statement for the load function, used by the UI to determine whether the load was successful.

meshroom/core/graph.py

@@ -267,7 +267,7 @@ def load(self, filepath, setupProjectFile=True, importProject=False, publishOutp
         """
         self._loading = True
         try:
-            self._load(filepath, setupProjectFile, importProject, publishOutputs)
+            return self._load(filepath, setupProjectFile, importProject, publishOutputs)
         finally:
             self._loading = False