**Is your feature request related to a problem? Please describe.**

If coin selection fails as part of `send` or `sendtoaddress`, the error message returned to the RPC client is `Insufficient funds`, even if the actual cause is due to `too-long-mempool-chain`.  This is likely to alarm users unnecessarily, as they will believe this means their funds have permanently vanished rather than being temporarily inaccessible.

**Describe the solution you'd like**

If coin selection fails due to `too-long-mempool-chain`, the `send` and `sendtoaddress` methods should pass that error back to the RPC client, instead of `Insufficient funds`.

**Describe alternatives you've considered**

It would be even nicer from a UX standpoint if the limit on mempool chain length were removed, but AIUI [that's a DoS vector](https://github.com/bitcoin/bitcoin/issues/9752#issuecomment-281464014), so is not going to happen.

**Additional context**

Apparently this is [already done for `sendrawtransaction`](https://github.com/bitcoin/bitcoin/pull/10015#issuecomment-288597075).  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

std::vector<CRecipient> vecSend;
    
    for (size_t idx = 0; idx < tx.vout.size(); idx++) {
        const CTxOut& txOut = tx.vout[idx];
        CRecipient recipient = {txOut.scriptPubKey, txOut.nValue, setSubtractFeeFromOutputs.count(idx) == 1};
        vecSend.push_back(recipient);
    }
    
    
    LOCK(wallet.cs_wallet);
    
    
    std::map<COutPoint, Coin> coins;
    for (const CTxIn& txin : tx.vin) {
        coins[txin.prevout]; 
    }
    wallet.chain().findCoins(coins);
    for (const CTxIn& txin : tx.vin) {
        const auto& outPoint = txin.prevout;
        if (wallet.IsMine(outPoint)) {
            
            coinControl.Select(outPoint);
        } else if (coins[outPoint].out.IsNull()) {
            error = _("Unable to find UTXO for external input");
            return false;
        } else {
            
            coinControl.SelectExternal(outPoint, coins[outPoint].out);
        }
    }