Light client: add better error handling

[rupam-04]

What type of PR is this?

Other

What does this PR do? Why is it needed?

It improves how we handle errors in a few scenarios:

• debug log instead of error log when finalized block is nil (nil block is expected at the beginning after checkpoint syncing)
• debug log instead of error log when there are too little sync committee bits set (it's not a Prysm error when there is too little participation)
• add block roots to logs for debugging purposes

Which issues(s) does this PR fix?

part of #12991

Other notes for review

Acknowledgements

• I have read CONTRIBUTING.md.
• I have made an appropriate entry to CHANGELOG.md.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.

[rkapka]

[diff]