pgsql: Added relation name in error messages for constraint checks.

Added relation name in error messages for constraint checks.

This gives more information to the user about the error and it makes such
messages consistent with the other similar messages in the code.

Reported-by: Simon Riggs
Author: Mahendra Singh and Simon Riggs
Reviewed-by: Beena Emerson and Amit Kapila
Discussion: https://postgr.es/m/CANP8+j+7YUvQvGxTrCiw77R23enMJ7DFmyA3buR+fa2pKs4XhA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/05f18c6b6b6e4b44302ee20a042cedc664532aa2

Modified Files
--------------
src/backend/commands/tablecmds.c | 21 ++++---
src/backend/executor/execMain.c | 5 +-
src/test/regress/expected/alter_table.out | 76 ++++++++++++-------------
src/test/regress/expected/copy2.out | 2 +-
src/test/regress/expected/create_table.out | 2 +-
src/test/regress/expected/create_table_like.out | 2 +-
src/test/regress/expected/domain.out | 8 +--
src/test/regress/expected/generated.out | 8 +--
src/test/regress/expected/identity.out | 4 +-
src/test/regress/expected/index_including.out | 4 +-
src/test/regress/expected/inherit.out | 6 +-
src/test/regress/expected/insert.out | 2 +-
src/test/regress/expected/privileges.out | 6 +-
src/test/regress/expected/reloptions.out | 4 +-
src/test/regress/expected/sequence.out | 2 +-
src/test/regress/expected/vacuum.out | 2 +-
src/test/regress/output/constraints.source | 4 +-
17 files changed, 82 insertions(+), 76 deletions(-)