pgsql: Fix ALTER TABLE...SET STATS error message for included columns

Fix ALTER TABLE...SET STATS error message for included columns

The existing error message was complaining that the column is not an
expression, which is not correct. Introduce a suitable wording
variation and a test.

Co-authored-by: Yugo Nagata <nagata(at)sraoss(dot)co(dot)jp>
Discussion: https://postgr.es/m/20180628182803.e4632d5a.nagata@sraoss.co.jp
Reviewed-by: Álvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/802b04cb3fadeb381001611f3ffa3c2762bdcc25

Modified Files
--------------
src/backend/commands/tablecmds.c | 23 +++++++++++++++--------
src/test/regress/expected/index_including.out | 14 ++++++++++++++
src/test/regress/sql/index_including.sql | 10 ++++++++++
3 files changed, 39 insertions(+), 8 deletions(-)