Backfill sync error logs due to insufficient custody peers

[eserilev]

During backfill sync if a node has no synced peers for a column they're required to custody, Lighthouse shows a bunch of ERROR messages. We should probably downgrade these log messages as the node will eventually find peers that can serve those columns and continue on with backfill sync.

[anshalshukla]

Hey @eserilev, looks like an easy first issue to take up. Please assign this to me.

[michaelsproul]

This also happens when backfilling beyond the DA window (#7751).