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@sraoss.co.jp>
Discussion: https://postgr.es/m/20180628182803.e4632d5a.nagata@sraoss.co.jp
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cb9db2ab0674381b92559e011738f19e2bd0f6f2

Modified Files
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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(-)