The message for a query with multiple COUNT(DISTINCT) currently looks like:
[localhost:21000] > select count(distinct x), count(distinct y) from t1;
ERROR: AnalysisException: all DISTINCT aggregate functions need to have the same set of parameters as count(DISTINCT x); deviating function: count(DISTINCT y)
How about adding an INFO message after the error saying something like:
INFO: If estimated counts are OK, replace COUNT(DISTINCT) with NDV(). Enable the APPX_COUNT_DISTINCT query option to perform this rewrite automatically.