Error msg to warn me not to give `set` queries a value

**Is your feature request related to a problem? Please describe.**

When I first use `set` operator in my query, I did not know it was null check (https://cube.dev/docs/query-format#set). So when I pass a `value` for this, I would expect an error message telling me this.

**Describe the solution you'd like**
If I query with `set` operator and I pass in a `value` with it, I would like Cube to return an error message telling me "Cannot use `value` with a `set` operator. `set` is a null check operator."

**Describe alternatives you've considered**
Making this clearer in the docs, but it can still easily be mistaken with a simple copy/paste mistake.

**Additional context**