pgsql: Improve two error messages related to foreign keys onpartitione

Improve two error messages related to foreign keys on partitioned tables

Error messages for creating a foreign key on a partitioned table using
ONLY or NOT VALID were wrong in mentioning the objects they worked on.
This commit adds on the way some regression tests missing for those
cases.

Author: Laurenz Albe
Reviewed-by: Michael Paquier
Discussion: https://postgr.es/m/c11c05810a9ed65e9b2c817a9ef442275a32fe80.camel@cybertec.at

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9c2a970d1fa4d30dc41d30d0b2ad557f40510549

Modified Files
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src/backend/commands/tablecmds.c          |  6 ++++--
src/test/regress/expected/foreign_key.out | 11 +++++++++++
src/test/regress/sql/foreign_key.sql      |  9 +++++++++
3 files changed, 24 insertions(+), 2 deletions(-)