feat: Add Custom Errors for Signer Management and Signature Validation in MultiSigContractV2

src/MultiSigContractV2.sol

@@ -29,4 +29,8 @@ contract MultiSigContractV2 is Ownable, ReentrancyGuard, Pausable {
     error MultiSigContract__TransactionExpired();
     error MultiSigContract__TransactionNotReady();
     error MultiSigContract__InvalidSignerCount();
+    error MultiSigContract__SignerAlreadyExists();
+    error MultiSigContract__SignerNotFound();
+    error MultiSigContract__InsufficientSignatures();
+    error MultiSigContract__InvalidAddress();
 }