chore: add an error field to "Finished building block" (#14696)

* fix: print "Finished building block" only when succeeded

* Add failed log

* Apply preston's review

CHANGELOG.md

@@ -33,6 +33,7 @@ The format is based on Keep a Changelog, and this project adheres to Semantic Ve
 - Added a Prometheus error counter metric for SSE requests.
 - Save light client updates and bootstraps in DB.
 - Added more comprehensive tests for `BlockToLightClientHeader`. [PR](https://github.com/prysmaticlabs/prysm/pull/14699)
+- Added an error field to log `Finished building block`.
 
 ### Changed
 

beacon-chain/rpc/prysm/v1alpha1/validator/proposer.go

@@ -103,6 +103,7 @@ func (vs *Server) GetBeaconBlock(ctx context.Context, req *ethpb.BlockRequest) (
 		"slot":               req.Slot,
 		"sinceSlotStartTime": time.Since(t),
 		"validator":          sBlk.Block().ProposerIndex(),
+		"err":                err,
 	}).Info("Finished building block")
 	if err != nil {
 		return nil, errors.Wrap(err, "could not build block in parallel")