util/errors: Extract CustomApiError struct

[Turbo87]

Instead of having a dedicated struct per HTTP status code (and then some more), we can use a single struct and save the status code inside the struct, next to the error message. This allows us to throw away a significant chunk of verbose code while the API behaves exactly the same as before. Admittedly this removes the ability for e.is::<NotFound>() checks, but we were not and probably shouldn't be using those anyway.

Note that a few structs were not converted to using this "generic" struct yet:

• InsecurelyGeneratedTokenRevoked because it is used in an e.is::<_>() check
• ReadOnlyMode because it is used in an e.is::<_>() check
• TooManyRequests because it is setting a custom header, which CustomApiError does not currently allow

Related:

• Use non-200 HTTP response status codes for API errors #7881

[LawnGnome]

LGTM.

Losing the ability to assert on specific errors is a little sad, but I assume we could add a way for tests to introspect CustomApiError if we really needed that for any of the errors that have been converted. It's probably not worth keeping the boilerplate that's getting removed here.

[Turbo87]

Losing the ability to assert on specific errors is a little sad

if there are cases where the introspection makes sense then I'm not against having custom error structs for that, but in 99% of our code base we don't need it :)