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.jpReviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>