Uninformative error from Attribute::parse_args for some expressions without "full" feature.

For most expressions that are not included without "full" feature
```
let attr: Attribute = parse_quote! {
#[precondition([a])]
};
let precondition: Expr = attr.parse_args().unwrap();
```
there is an error `Error("unsupported expression; enable syn's features=[\"full\"]")`.

But for `Assign` expressions `#[precondition(value = 5)]` there is an uninformative `Error("unexpected token")`.