Expose error messages strings via function in h3api.h

Expose error message strings (which are currently given as comments in h3api.h), mainly so that the bindings can forward those explanations to users, without having to manually copy the messages.

Some examples in other projects:

• SDL: https://wiki.libsdl.org/SDL_GetError
• Sqlite: https://sqlite.org/c3ref/errcode.html

Other notes:

• Also needs to be added to our documentation
• probably English only for now
• const char *errorToString(H3Error err);
• Eventually we could have the website build the error table from this function too