Enable nilerr linter & fix findings

[jtraglia]

Enable the nilerr linter in golangci-lint and fix a few of its findings, which I believe are minor but valid.

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

There are four calls to ValidatorAtIndex that will return false, nil instead of false, err. Like this:

prysm/beacon-chain/core/helpers/sync_committee.go

Lines 37 to 40 in ff1b03a

	val, err := st.ValidatorAtIndex(valIdx)
	if err != nil {
	return false, nil
	}

I'm not sure if this is error state is even reachable, but just in case it is, I think we should return err.

In other places, this is handled as expected. Like this:

prysm/beacon-chain/core/blocks/signature.go

Lines 76 to 79 in ff1b03a

	proposer, err := beaconState.ValidatorAtIndex(proposerIndex)
	if err != nil {
	return err
	}

But if returning nil there is intended, I think it's worthy of a comment.

---

Edit: this also fixes some findings from the CI lint check (I didn't see these locally).

For one's that seemed fine, I added: //nolint:nilerr

[terencechain]

thanks!

[jtraglia]

Hmm the CI has more findings of nilerr. I didn't see those locally. Will check them out.

[prestonvanloon]

This is failing some unit tests. Possibly due to poor test setup.