Raise the most internal exception when parsing nested dataclasses

[hellais]

Currently when an exception happens in a nested dataclass object, the caller will only get context about the most high level parent dataclass which triggered the exception. This makes it tricky to understand the root cause of the validation issues.

When a field is invalid, we re-raise the child exception so it's easier to pin-point exactly where it occurred.

[Fatal1ty]

Hey @hellais

I don't like this idea because it will make the context of the outer scope missing. It'll be hard to guess which of the two fields "a" and "b" has got an invalid value:

@dataclass
class Inner(DataClassDictMixin):
    x: int

@dataclass
class Outer(DataClassJSONMixin):
    a: Inner
    b: Inner

obj = Outer.from_dict(
    {
        "a": {"x": 1},
        "b": {"x": "invalid"},
    }
)

This makes it tricky to understand the root cause of the validation issues.

You can read the traceback to find the root cause actually. Given your test case that will be the following:

mashumaro.exceptions.InvalidFieldValue: Field "child" of type MyDataClassWithChild in DataClass has invalid value {'child': {'a': 'invalid', 'b': 1}}
mashumaro.exceptions.InvalidFieldValue: Field "child" of type MyDataClass in MyDataClassWithChild has invalid value {'a': 'invalid', 'b': 1}
mashumaro.exceptions.InvalidFieldValue: Field "a" of type int in MyDataClass has invalid value 'invalid'
ValueError: invalid literal for int() with base 10: 'invalid'

[Fatal1ty]

I think it could be better save a reference to the original exception that caused InvalidFieldValue.