ChainProcessor returns when head is not found

[NullSoldier]

Summary

This is experimenting with a fix for a problem i've seen. If the wallet gets ahead of the chain, the node will no longer start with this error.

Error: Chain processor head not found in chain: 0000000000012f4aca795fe01d3823b6d2b01a581fb7b4d41e78c09e68f477ba
    at Function.isNotNull (/ironfish/src/assert.ts:26:13)
    at ChainProcessor.update (/ironfish/src/chainProcessor.ts:76:12)
    at <anonymous> (/ironfish/src/wallet/scanner/walletScanner.ts:132:26)

You can reproduce this by running

• ironfish chain:rewind 665566 // use a sequence 1 behind the head
• ironfish start

The problem is that the ChainProcessor crashes if it cannot find the head in the chain. After looking at all the usages of ChainProcessor, I think instead it should just return having not processed any blocks.

Testing Plan

Documentation

Does this change require any updates to the Iron Fish Docs (ex. the RPC API
Reference)? If yes, link a
related documentation pull request for the website.

[ ] Yes

Breaking Change

Is this a breaking change? If yes, add notes below on why this is breaking and label it with breaking-change-rpc or breaking-change-sdk.

[ ] Yes