chore: stmgr: migrations: do not log noisily on cache misses

[arajasek]

Related Issues

@rjan90 reported that he was seeing this error when testing nv21 on devnets:

2023-07-13T13:37:17.071+0200    ERROR   statemgr        stmgr/forks.go:184      failed to lookup previous migration result      {"err": "error loading migration result: datastore: key not found", "errVerbose": "error loading migration result:\n    github.com/filecoin-project/lotus/chain/stmgr.(*migrationResultCache).Get\n        /Users/phi/lotus/chain/stmgr/stmgr.go:96\n  - datastore: key not found"}

This error is benign (just reporting a cache miss), and so shouldn't be a loud failure.

Proposed Changes

Drop this error to a Debug if it is key not found (cache miss).

Additional Info

Checklist

Before you mark the PR ready for review, please make sure that:

• Commits have a clear commit message.
• PR title is in the form of of <PR type>: <area>: <change being made>
  • example: fix: mempool: Introduce a cache for valid signatures
  • PR type: fix, feat, build, chore, ci, docs, perf, refactor, revert, style, test
  • area, e.g. api, chain, state, market, mempool, multisig, networking, paych, proving, sealing, wallet, deps
• New features have usage guidelines and / or documentation updates in
  • Lotus Documentation
  • Discussion Tutorials
• Tests exist for new functionality or change in behavior
• CI is green