COUNT DISTINCT usability improvement

Currently we have `citus.count_distinct_error_rate` GUC. If we set this variable to something other than 0, it switches to use hll for count distinct queries.
The problem here is that, even if the extension is not loaded we allow him to set this variable. This is fine, but if the user is not aware that he needs hll when he is setting the GUC, his count distinct queries will error out. Additionally, the error message does not give him a clue that he can run count distinct if he sets the GUC back to 0.

I think it is better if we warn him if the extension is not loaded in the set phase or doesn't allow him to set it at all. Alternatively, we may extend the warning message he gets when he runs the query and advice him to set the citus.count_distinct_error_rate to 0.