`FindValidL1URL` does not handle 403 Forbidden properly

[geoknee]

1. It will continue, whereas this is a fatal error
2. It will report "l1 has mismatched chain ID" which we can't know because we didn't get a responsehttps://app.circleci.com/pipelines/github/ethereum-optimism/superchain-registry/15762/workflows/a2290924-0c1e-4405-91fb-b52b5c37723d/jobs/96919

superchain-registry/ops/internal/config/superchain.go

Lines 41 to 42 in adf8cf6

	if err := validateL1ChainID(ctx, url, superchainId); err != nil {
	lgr.Warn("l1-rpc-url has mismatched l1 chainId", "urlIndex", i, "err", err)