Fix error message returned by DDL request, in case index service is not configured

As discussed in the comments on MB-60646, a wrong error message will be returned to the user on DDL requests.

The original root cause of this problem is: indexing code does not check for availability of index service nodes in the cluster - before returning error.

A potential fix: Return error during NewGSIIndexer if there aren't any index service nodes configured. Note: need to handle the case where all index service nodes are failed over.