Flatten error messages

[drhagen]

I have come to like the the flattened nature of the errors in Pydantic. The use of loc instead of nested maps works better in the face of lists, whose keys are integers and cannot be put into JSON objects without being stringified. The data structure also allows for more information to be returned other than the message, which is a good thing.

I am thinking a data structure like this would be good for DeserializationErrors to serialize into:

[
  {
    "location": ["models", 5],
    "type": "ModelNotFoundError",
    "context": {
      "actual": "mouse_pk"
    },
    "message": "Model 'mouse_pk' not found"
  },
  {
    "location": ["data", 101, "measurement_unit"],
    "type": "ParseError",
    "context": {
      "line": 1,
      "character": 4,
      "expected": ["r'[A-Za-z]+'"],
    },
    "message": "Expected r'[A-Za-z]+' but found '/'\nLine 1, character 4\n\nmg//kg\n   ^"
  }
]

At the same time, DeserializationError should either gain lots of functionality to assist with building errors or there should be a Builder class that assists with this and can be converted to a DeserializationError.