Fixup: Twelve days of AoC

[xsebek]

• Use simple string literal, const &str is hard 😞
• Fixup error message and comments
• Fixup month number 12

[xsebek]

Hi @Lollyde! I saw your PR fspoettel#82 and wanted to use it in my repo this year.

I noticed you replaced the 12 month of December by days!() macro, so I fixed that. I also wanted to improve the error messages, but as you said, constant strings are hard in Rust. 😩

Good luck getting your changes merged upstream! 👍

[Lollyde]

...hhuh, youre right, i did mess up the today command! turns out it just happens to work for any configurations past 2024. note to self, do not write code and open a pr in a hurry like this