Log error on commit construction

[AnnaShaleva]

It's an open question what should we do when the node failed to sign the block.

[codecov]

Codecov Report

Attention: Patch coverage is 30.43478% with 16 lines in your changes missing coverage. Please review.

Project coverage is 57.86%. Comparing base (026852c) to head (fe0edd5).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
send.go	27.27%	12 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
- Coverage   58.25%   57.86%   -0.40%     
==========================================
  Files          33       33              
  Lines        2271     2283      +12     
==========================================
- Hits         1323     1321       -2     
- Misses        863      873      +10     
- Partials       85       89       +4     

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[AnnaShaleva]

what should we do when the node failed to sign the block.

@roman-khimov what do you think? It's related to bane-labs/go-ethereum#396 (comment).

Right now the node creates an empty PreCommit and relays. It causes PreCommit verification errors on other nodes. We may avoid that by considering PreCommit/Commit creation error as critical for the node's functioning.

[roman-khimov]

Ideally, log (as an error) and do not send anything. Yeah, BFT is cool, but we don't want to send payloads that are broken by definition.

Logic is changed.