Provide more useful info than "No checkpoints provided."

[mikegerber]

From one of @kba's conversations in Gitter:

"checkpoint": "/home/najem/testa/calamari_model/*.ckpt.json"
Exception: No checkpoints provided.

I see this error from time to time and it would be lot clearer for the users if we could help with messages like:

• "File /home/najem/testa/calamari_model/*.ckpt.json not found"
• "/home/najem/testa/calamari_model/*.ckpt.json" are Calamari 0.3 models, we need Calamari 1.0 models