fix(api-server): improve error handling and return status

[lahabana]

• never return 412 (it was never appropriate
• create better error hierarchy to simplify handler
• use in some places errors.Is

Checklist prior to review

• Link to relevant issue as well as docs and UI issues --
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as a image registry) and it will work on Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS --
• Tests (Unit test, E2E tests, manual test on universal and k8s) --
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? (this GH action will add "backport" label based on these file globs, if you want to prevent it from adding the "backport" label use no-backport-autolabel label) --
• Do you need to explicitly set a > Changelog: entry here or add a ci/ label to run fewer/more tests?

[jakubdyszkiewicz]

Nice, I like this direction very much. Our error handling bothers me. It would be nice to create a set of rules as we have for logging https://github.com/kumahq/kuma/blob/master/docs/madr/decisions/003-logging.md
to be more specific:

• When to create a separate type, when to just do errors.Wrap()
• What functions do you need to define for err
• What should be the name of the error type and function to create an error

I'm up to writing this down.

[lahabana]

@jakubdyszkiewicz I think it makes sense to have a MADR (also because implementing things like Is is not always straightforward :)

[bartsmykla]

/golden_files