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
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