pydantic: Errors format is too complex

Hi,

I would like to discuss errors format, to my mind it’s too complex to understand and parse for now.

1. Let’s start from something simple

class Test(BaseModel):
    k_str: constr(min_length=42)
    k_int: int

Test(k_str='min')

1.1 Error codes only

{
    "k_str": "min_length",
    "k_int": "required",
}

1.2 Error codes + human readable messages

{
    "k_str": {
        "error_code": "min_length",
        "error_message": "...",
    },
    "k_int": {
        "error_code": "required",
        "error_message": "...",
    },
}

I prefer variant 1.1 and in all examples below I will use it.

1. Recursive models

class Foo(BaseModel):
    count: int

class Bar(BaseModel):
    uid: uuid.UUID

class Spam(BaseModel):
    foo: Foo
    bars: List[Bar]

Spam.parse_obj({
    'bars': [
        uuid.uuid4(),
        'not_uuid',
    ],
})

2.1 Error codes only

{
    "foo": {
        "count": "required",
    },
    "bars": {
        1: {
            "uid": "invalid",
        },
    },
}

Some points about format:

1. We need error_code to have ability easily write error parsers, it’s very useful for APIs and documentation. We can extend error codes and add more context, e.g.: min_length:42 where 42 is required minimum.
2. We need error_message in some cases for humans, but I don’t think that you wound like to think about localization… Also I think any developer can build their own human readable messages using error codes only.
3. I don’t think that we need to return list of errors for one type, let’s return first one. It’s more easily to implement and support and it’s better for performance reasons.
4. Do not use track attr for recursive models, it’s complex to parse. Let’s use tree logic and dictionaries can help us with this. Lists can be represents as dictionaries too, just use element index as key.
5. Using my proposal you can easily convert errors dictionary to something else e.g. to JSONPoiters https://tools.ietf.org/html/rfc6901.
6. This format isn’t my idea, something very similar used in Cerberus.

I know that changes like this are backward incompatible, so let’s discuss and let me know if you need more detailed examples. Also I can try to implement proof of concept and send PR to you.