Refactor errors, adding section for Message Function Errors

[eemeli]

CC @mradbourne, who should be added to the repo.

As discussed yesterday, our error categories could be improved. This PR effects the categorisation we discussed, with "Message Function Error" introduced as the supertype of the errors that are coming from a custom function and "Invalid Expression" renamed to avoid its re-definition of what "valid" means in an MF2 context.

This and #767 should not be considered as blockers for each other. Instead, we'll need a fixup PR after both of these land adding the kebab-case error codes to errors.md and updating the ones used in the tests.