Move custom panic hook function to the crate root

[FenrirWolf]

Setting the ctru panic hook can be pretty useful even if you don't intend to do anything else with the error applet, and I suspect that most users don't realize that this function exists at all. So I figured we might as well move the function to a more conspicuous location.

[Meziu]

I'm ok with the transfer, since it should be clear that the hook uses the error applet in the documentation.