Better message for nested TypedDict

An attempt to define a nested TypedDict results in a not very helpful message:

```py
from mypy_extensions import TypedDict

D = TypedDict('D', {
    'x': TypedDict('E', {  # Invalid field type
        'y': int
    })
})
```

A better message could be something like "Inline TypedDict types not supported; use assignment" statement to define a TypedDict".
