Refactor UpstreamEndpoints retain to return enum

[markmandel]

quilkin/src/config/endpoints.rs

Line 83 in c2df710

pub fn retain<F>(&mut self, predicate: F) -> Result<(), AllEndpointsRemovedError>

We want to change the function signature to return an enum rather than a Result so that it doesn't seem like something bad has happened when we catch an error.

Maybe something like:

enum RetainResult {
   Empty
   Ok
}

⬆️ Sacrificial draft. Change as necessary.

Then we can also add extra states if needed down the line.

[iffyio]

Also lets include must_use in the enum to enforce that the caller doesn't ignore the returned result since that can otherwise introduce bugs in their code if e.g the call rejected the operation while they assumed it succeeded.
https://doc.rust-lang.org/reference/attributes/diagnostics.html#the-must_use-attribute