chore(index-registry): add QuorumAlreadyExists error

[itsmehdiabdi]

Motivation:

The existing logic used the QuorumDoesNotExist error even in the case when a quorum already existed. This was misleading and did not properly reflect the error scenario. Accurate error handling is essential for debugging and for ensuring clarity during interactions with the contract.

Modifications:

• Introduced a new custom error QuorumAlreadyExists in the IndexRegistryErrors interface.
• Updated the initializeQuorum function in IndexRegistry to use the new QuorumAlreadyExists error when a quorum is already initialized.

Result:

• The error handling logic is now more precise.
• Developers and users will get clearer feedback when attempting to initialize an already existing quorum.