pgsql: Clean up error messages related to bad datetime units.

Clean up error messages related to bad datetime units.

Adjust the error texts used for unrecognized/unsupported datetime
units so that there are just two strings to translate, not two
per datatype. Along the way, follow our usual error message style
of not double-quoting type names, and instead making sure that we
say the name is a type. Fix a couple of places in date.c that
were using the wrong one of "unrecognized" and "unsupported".

Nikhil Benesch, with a bit more editing by me

Discussion: https://postgr.es/m/CAPWqQZTURGixmbMH2_Z3ZtWGA0ANjUb9bwtkkxSxSfDeFHuM6Q@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4b160492b9b8345747dfcad468eb89e9ccc0a876

Modified Files
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src/backend/utils/adt/date.c | 36 +++++++------
src/backend/utils/adt/timestamp.c | 99 ++++++++++++++--------------------
src/test/regress/expected/date.out | 18 +++----
src/test/regress/expected/interval.out | 4 +-
src/test/regress/expected/time.out | 6 +--
src/test/regress/expected/timetz.out | 4 +-
6 files changed, 76 insertions(+), 91 deletions(-)