some issues in the GetLightClientUpdatesByRange function

[rupam-04]

💎 Issue

Background

I am an EPF5 fellow who has been talking with @rkapka for some while about light clients and he pointed out some errors in the code. Listing them below

Description

• We don't want to skip the update here
• In https://github.com/prysmaticlabs/prysm/blob/a8ecf5d11878324690115c612beefd66ac6386dd/beacon-chain/rpc/eth/light-client/handlers.go#L147-#L190 all the places where err != nil, they should be returning httputil.HandleError
• Here 66% votes are not required, 1 is enough. Also relevant unit tests required to be added

[rkapka]

Closing in favor of #12991