Give path when checkpoint can't be found (facebookresearch#1040)

Summary:
Pull Request resolved: facebookresearch#1040

Pull Request resolved: fairinternal/fairseq-py#836

Reviewed By: myleott, liezl200

Differential Revision: D16889252

fbshipit-source-id: 45a1b6c1217fb099f0350096e38e1c7d83ea0a64

fairseq/trainer.py

@@ -150,8 +150,8 @@ def load_checkpoint(
                 self.get_model().load_state_dict(state['model'], strict=True)
             except Exception:
                 raise Exception(
-                    'Cannot load model parameters from checkpoint, '
-                    'please ensure that the architectures match.'
+                    'Cannot load model parameters from checkpoint {}; '
+                    'please ensure that the architectures match.'.format(filename)
                 )
 
             extra_state = state['extra_state']