Catch old models error and display it nicely

Traceback:

File "/usr/src/app/apps/core/tasks.py" in transcribe
  525.         part.transcribe(model=model)

File "/usr/src/app/apps/core/models.py" in transcribe
  590.             model_ = kraken_models.load_any(model.file.path)

File "/usr/local/lib/python3.7/site-packages/kraken/lib/models.py" in load_any
  157.     seq = TorchSeqRecognizer(nn, train=train, device=device)

File "/usr/local/lib/python3.7/site-packages/kraken/lib/models.py" in __init__
  52.         if nn.model_type not in [None, 'recognition']:

File "/usr/local/lib/python3.7/site-packages/kraken/lib/vgsl.py" in model_type
  471.         return self.user_metadata['model_type']

Exception Type: KeyError
Exception Value: 'model_type'
Request data not supplied

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