Improve error log when index does not exist

[LuisNaldo7]

Issue

Wrong error message when validator index does not exist. This leeds to logs not being shown for valid indices aswell.
Release 0.2.0

Steps to reproduce

Set validator index to some high value like 900000

Output

13-01-2023 14:53:39 - [ERROR | protocol.request.send_beacon_api_request():] Couldn't read from beacon client. Retry in 5 seconds.

Expected output

Some message that does not indicate a broken beacon endpoint.