Fix some failed precommit handling

[arajasek]

I think this is correct?

Thanks to @Arctic in the Filecoin slack for the report.

[magik6k]

Can you share more context on this issue?

[arajasek]

@magik6k I think it's just a question of whether this is the correct thing to be doing.

• Is the boolean returned by checkPreCommitted an indication of whether the sector has already precommitted? If so, I think we should be returning false in the error cases, and true otherwise?

• If I'm understanding it correctly, the log.Warn("sector %d is precommitted on chain, but we don't have precommit message", sector.SectorNumber) should be triggered if we didn't have the PreCommitMessage field in the SectorInfo (that is if it is nil, not if it isn't).

Might be wrong on both counts 🤷‍♀️

[whyrusleeping]

@magik6k context is here: https://filecoinproject.slack.com/archives/C0179RNEMU4/p1598926390173100

someone claims theyve found a silly bug but doesnt really say what it is. they seem really intent on being 'helpful'

[magik6k]

This fix looks correct, not sure what how this code ended up in this shape