message: Don't error out when a range has already been computed
  When a range of blocks has already been computed, use that existing data
  instead of erroring out. This is used in the zPoS pipeline where, after
  precomputation, the passed `pindexCheckpoint` is likely to be at a
  block height below the accumulator's tip/end.
diff: @@ -570,10 +570,9 @@ bool GenerateAccumulatorWitness(CoinWitnessData* coinWitness, AccumulatorMap& ma
  nHeightStop = nHeightMax;
  }
  - if (nHeightStop <= coinWitness->nHeightAccEnd)
  - return error("%s: trying to accumulate bad block range, start=%d end=%d", __func__, coinWitness->nHeightAccEnd, nHeightStop);
  -
  + if (nHeightStop > coinWitness->nHeightAccEnd)
  AccumulateRange(coinWitness, nHeightStop - 1);
  +
  mapAccumulators.Load(chainActive[nHeightStop + 10]->nAccumulatorCheckpoint);
  coinWitness->pWitness->resetValue(*coinWitness->pAccumulator, *coinWitness->coin);
  if(!coinWitness->pWitness->VerifyWitness(mapAccumulators.GetAccumulator(coinWitness->denom), *coinWitness->coin))
  