Fix error message for COMMENT/SECURITY LABEL ON COLUMN xxx IS 'yyy'

Fix error message for COMMENT/SECURITY LABEL ON COLUMN xxx IS 'yyy'

When the column name is an unqualified name, rather than table.column,
the error message complains about too many dotted names, which is
wrong.  Report by Peter Eisentraut based on examination of the
sepgsql regression test output, but the problem also affects COMMENT.
New wording as suggested by Tom Lane.