Verify Atlas Search Index exists

[wbleonard]

A very obscure exception occurs if the Atlas Search index doesn't exist.

OperationFailure: PlanExecutor error during aggregation :: caused by :: Error connecting to localhost:28000 (127.0.0.1:28000) :: caused by :: Connection refused, full error: {'ok': 0.0, 'errmsg': 'PlanExecutor error during aggregation :: caused by :: Error connecting to localhost:28000 (127.0.0.1:28000) :: caused by :: Connection refused', 'code': 6, 'codeName': 'HostUnreachable', '$clusterTime': {'clusterTime': Timestamp(1701962312, 7), 'signature': {'hash': b'\xb7Fx\xee\x01L\xa4%\xbb\r\x8d\x9f\x04\x8b\xbcW\x07\xa9\x8f\xee', 'keyId': 7306191798055993350}}, 'operationTime': Timestamp(1701962312, 7)}

Either validate it exists OR catch the exception and provide a clearer response.