message: Change error message from PoW to generic difficulty value
diff: @@ -3679,7 +3679,7 @@ bool ContextualCheckBlockHeader(const CBlockHeader& block, CValidationState& sta
  const int nHeight = pindexPrev == NULL ? 0 : pindexPrev->nHeight + 1;
  // Check proof of work
  if (block.nBits != GetNextWorkRequired(pindexPrev, &block, consensusParams,block.IsProofOfStake()))
  - return state.DoS(100, false, REJECT_INVALID, "bad-diffbits", false, "incorrect proof of work");
  + return state.DoS(100, false, REJECT_INVALID, "bad-diffbits", false, "incorrect difficulty value");
  // Check timestamp against prev
  if (pindexPrev && block.IsProofOfStake() && block.GetBlockTime() <= pindexPrev->GetMedianTimePast())
  