gh-109798: Normalize `_datetime` and `datetime` error messages (#127345)

Updates error messages in datetime and makes them consistent between Python and C.

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Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Paul Ganssle <1377457+pganssle@users.noreply.github.com>