Better error handling if trainer config doesn't contain "default" section

[chriselion]

• Allow an empty "default" section.
• Raise an exception if both "default" and the brain name sections are missing
• Raise an exception if the "trainer" key is missing.
• Make the unknown trainer type message a bit clearer (put the unknown type in the message)

Open question: do we want to support this still?

ml-agents/ml-agents/mlagents/trainers/trainer_util.py

Lines 94 to 96 in ff0edb4

	_brain_key: Any = brain_name
	while not isinstance(trainer_config[_brain_key], dict):
	_brain_key = trainer_config[_brain_key]

Seems like it was there for compatibility at some point.

[ervteng]

👍