Incorrect error message for valid match/case {**rest,} syntax

This code generates an error:
```python
match x:
    case {"a": 1, **keys,}:
        print("hello")
```
produces: `CSTValidationError("Cannot have a trailing comma without **rest")`

The error shouldn't happen, since this is valid syntax.

BTW: the error message is also inverted, it's trying to say "Cannot have a trailing comma with **rest"