feat: itests: add logs to blockminer.go failure case

[arajasek]

Related Issues

example failure that's hard to investigate due to missing info: https://circleci.com/api/v1.1/project/github/filecoin-project/lotus/970891/output/104/0?file=true&allocation-id=645c52859e67117b20aa0012-0-build%2F4A3521EB

Proposed Changes

Spell out which block height was missed by the node, and what height the node is at instead

Additional Info

Checklist

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• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
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• Tests exist for new functionality or change in behavior
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