feat: add error codes to exceptions

[braindigitalis]

This lets users check what type of exception by exception.code() instead of having to rely on string comparison of exception.what(). This is a non-breaking change as the previous functionality still exists alongside the old style constructors for anyone who is directly instantiating their own dpp exceptions in their bot (not sure why they would be, though).

Note the huge enum exception_error_code in exception.h. This will need to be maintained periodically by us as Discord add new errors. It can also be used with get_error() of a callback and matched against get_error().code.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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