Standard error message when trying to exit an exited validator

[nalepae]

Describe the bug When using https://goerli.beaconcha.in/tools/broadcast with the following payload, corresponding to an already exited validator on Goerli:

{
"message": 
    {
        "epoch": "162304",
        "validator_index": "384413"
    },
    "signature": "0x910c7cd56987115b0d3e51cddc9c94a25f7ba4949b7343581e853f58ec5e0ef8a7bf1fb61fe24a4210ca0f37a50987630543e7b21e4d2e8becf9e1dfa1a239b2988f48e871053c336d890e162846c634cceef39bc9744e7dfd398ccfacd9eccd"
}

We get the following error:

It could be nice to have a more specific message like: Not active validator.

For information, here is the Lighthouse beacon node response in such a case:

{
    "code": 400,
    "message": "BAD_REQUEST: Invalid object: gossip verification failed: ExitValidationError(Invalid(NotActive(384413)))",
    "stacktraces": []
}

The Lighthouse's message is pretty clear!