Built against `master` branch 801dd40666d1e6009920ad3ff755c7bb993b2a62:

If I have no wallet.dat file and run:

    bitcoind -usehd=0

A new wallet.dat file is created and an HD seed is assigned.

I see an error message in the source:

    "Error loading %s: You can't disable HD on an already existing HD wallet or create new non-HD wallets."

but that doesn't get shown because `fFirstRun` is true, and so the code which checks for `-usehd` values doesn't run.

If it's no longer possible to create non-HD wallets, specifying -usehd=0 when creating a wallet should be an error, and not get silently ignored.
{
    AssertLockHeld(cs_main);
    AssertLockHeld(cs_wallet);
    
    
    
    CBlockIndex* const startBlock = chainActive.FindEarliestAtLeast(startTime - TIMESTAMP_WINDOW);
    LogPrintf("%s: Rescanning last %i blocks\n", __func__, startBlock ? chainActive.Height() - startBlock->nHeight + 1 : 0);
    if (startBlock) {
        const CBlockIndex* const failedBlock = ScanForWalletTransactions(startBlock, update);
        if (failedBlock) {
            return failedBlock->GetBlockTimeMax() + TIMESTAMP_WINDOW + 1;
        }
    }
    return startTime;
}
/**
 * Scan the block chain (starting in pindexStart) for transactions
 * from or to us. If fUpdate is true, found transactions that already
 * exist in the wallet will be updated.
 *
 * Returns null if scan was successful. Otherwise, if a complete rescan was not
 * possible (due to pruning or corruption), returns pointer to the most recent
 * block that could not be scanned.
 */
CBlockIndex* CWallet::ScanForWalletTransactions(CBlockIndex* pindexStart, bool fUpdate)