Improved error propagation for fallible validator api queries

[jstuczyn]

Description

Closes: #1957

The change is best represented with the following code snippet:

let client = ApiClient::new("https://qa-validator-api.nymtech.net/api/".parse().unwrap());
let mix_id_that_doesnt_exist = 123456;
let res = client
    .get_mixnode_stake_saturation(mix_id_that_doesnt_exist)
    .await;
println!("{:?}", res)

// old: Err(ValidatorAPIError { source: ReqwestClientError { source: reqwest::Error { kind: Decode, source: Error("expected value", line: 1, column: 1) } } })
// new: Err(ValidatorAPIError { source: ApiRequestFailure { status: 404, error: RequestError { message: "mixnode bond not found" } } })

Note: it requires both, validator-api redeployment and local client updates

Checklist:

• added a changelog entry to CHANGELOG.md