pgsql: Clarify use of "statistics objects" in the code

Clarify use of "statistics objects" in the code The code inconsistently used "statistic object" or "statistics" where the correct term, as discussed, is actually "statistics object". This improves the state of the code to be more consistent. While on it, fix an incorrect error message introduced

pgsql: Clarify use of "statistics objects" in the code

Clarify use of "statistics objects" in the code The code inconsistently used "statistic object" or "statistics" where the correct term, as discussed, is actually "statistics object". This improves the state of the code to be more consistent. While on it, fix an incorrect error message introduced