Conduit does not log exporter callback errors

[urtho]

exporterHandler should log errors from failed callbacks but uses incorrect variable for that.

this little patch solves it

@@ -631,7 +631,7 @@ func (p *pipelineImpl) exporterHandler(exporter exporters.Exporter, blkChan plug
                                        if callbackErr != nil {
                                                p.logger.Errorf(
                                                        "exporter %s # round %d failed callback #%d but CONTINUING to NextRound=%d: %v",
-                                                       eName, lastRound, i, nextRound, lastError,
+                                                       eName, lastRound, i, nextRound, callbackErr,
                                                )
                                        }
                                }