pgsql: Message style improvements

Message style improvements

An improvement pass over the new stats import functionality.

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/80f11061323499ef2d07af32ae993473115ca176

Modified Files
--------------
src/backend/statistics/attribute_stats.c   | 25 +++++++------
src/backend/statistics/relation_stats.c    |  2 +-
src/backend/statistics/stat_utils.c        | 14 +++----
src/test/regress/expected/stats_import.out | 60 +++++++++++++++---------------
4 files changed, 52 insertions(+), 49 deletions(-)