Improve error message when using optional types in `bounced<>`

[Gusarich]

The current compiler error message when using an optional message type (MyMsg?) inside the bounced<> constructor is imprecise and unclear.

Minimal reproducible snippet:

message MyMsg {}

contract TestContract {
    bounced(msg: bounced<MyMsg?>) {
        // Do nothing
    }
    receive() {}
}

Current compiler error:

Error: Only named type can be bounced<>

Suggested improved error message:

Error: Optional types cannot be used with bounced<>, remove '?' from 'MyMsg?'

Explanation:
The existing error message incorrectly implies the issue is about naming, rather than optionality. A clearer error message explicitly indicating the problem is due to using an optional type would better guide developers in resolving this issue.

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LLM Fuzzing discovery (see #2490)