Refactor all error-related code into new module (`vcard_errors`)

Created by: Zearin

@L0b0 : I remember that you have very high standards for your repo history, so let me know if I can rewrite any of these commits so that they’re more to your liking.

I’m still not an expert, but I’ve been learning how to use git rebase --interactive for simple history clarification, squashing similar commits together, etc. Since it’s easier to squash lots of small commits than it is to split a few large commits, I was doing my best to make these commits as small as possible. Maybe with enough work some “best practices” for commits will start to sink in… ;)