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See merge request KartKrew/Kart!8

Also, correct a minor copypaste failure in one of the I_Errors' messages.

* Wrap behind-the-scenes `addfile/exec` contents in quotation marks to allow for files with spaces in them.

* Make the I_Errors in filesrch.c more descriptive.
* Fix up the loading of sounds.kart such that it has SOME protection against being loaded with game-modifying lumps.
* SERIOUSLY fix up the MD5/mainwads++ section in D_SRB2Main, since 1) I broke it and 2) it was already a little broken in the first place but we just didn't notice it because #ifndef DEVELOP.

Bitch.