Synchronizer error handling about zkopru

[vimwitch]

The synchronizer makes lots of calls to a web3 node that can potentially error, like here and here. These calls should be protected by some try/catch/retry logic to make sure that synchronization continues if there is a brief disruption to the connection.

There's also lots of repeated logic in the synchronizer around loading events from the past, this can probably be refactored at the same time.