Document and hardcode error/warning IDs

[trillllian]

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error and warning ids are supposed to be unchanging. and it's supposed to be easy to determine what an error is from its code. currently, the only thing that guarantees the ids being static is their order in the source code. this makes it both easy to accidentally change the ids of some warnings/errors and very annoying to figure out what a specific id refers to without having access to some code that throws it. we should document all currently used error and warning ids in the manual, and also change the code so the ids are right next to the descriptions.