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