pgsql: Improve wording of two error messages related to generated colum

Improve wording of two error messages related to generated columns.

Clarify that you can "insert" into a generated column as long as what
you're inserting is a DEFAULT placeholder.

Also, use ERRCODE_GENERATED_ALWAYS in place of ERRCODE_SYNTAX_ERROR;
there doesn't seem to be any reason to use the less specific errcode.

Discussion: https://postgr.es/m/9q0sgcr416t.fsf@gmx.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d36228a9fcdccd57a7dc332572eb9837c7c301e6

Modified Files
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src/backend/rewrite/rewriteHandler.c          | 17 ++++++++++-------
src/test/regress/expected/generated.out       | 20 ++++++++++----------
src/test/regress/expected/identity.out        | 12 ++++++------
src/test/regress/expected/updatable_views.out |  4 ++--
4 files changed, 28 insertions(+), 25 deletions(-)