Provide useful information when backward sync preconditions not met

[ajsutton]

Description

When syncing a new teku+besu node, besu 22.4.0 wound up repeatedly logging:

2022-07-20 21:00:12.578+00:00 | ForkJoinPool.commonPool-worker-1 | INFO  | BackwardsSyncAlgorithm | Waiting for preconditions...

This is an entirely unhelpful log message. It should clearly explain what preconditions it's waiting on so the user knows why sync is not progressing. Looking at the code in this case I think it was that the initial sync hadn't completed.

[iamhsk]

@daniellehrner This might be useful for our logging updates

[non-fungible-nelson]

@gezero as well RE backwards sync.

[non-fungible-nelson]

Closing for backwards sync changes in 22.10.x