[RTL7h] Throw exception for optional callback

[sacOO7]

• RTL7h is updated and merged as per https://github.com/ably/specification/pull/209/files
• If optional callback is provided when attachOnSubcribe is false, subscribe should throw error.
• Current implementation calls optional callback with null error.

┆Issue is synchronized with this Jira Task by Unito

[sacOO7]

Fixed via #1043