Assignment of Structured Config to Subscripted Dict raising wrong error.

[pereman2]

Describe the bug
Assigning a structured config into a Subscripted Dict error message is not representing correctly the action.
To Reproduce

@dataclass
class DictClass:
    foo: Dict[str, int] = field(default_factory=lambda: {"a": 4})

@dataclass
class User:
    age: int = 4
    name: str = "foo"
cfg = OmegaConf.structured(DictClass)
cfg.foo = User()
ValidationError: Invalid type assigned : str is not a subclass of int. value: foo
        full_key: foo.name
        reference_type=Dict[str, int]
        object_type=dict

Expected behaviour
Structured config assignment to subscripted dicts should't be allowed and should have an descriptive error message.

Additional context

• OmegaConf version: 2.1.0
• Python version: 3.8
• Operating system : macOS
• Please provide a minimal repro