**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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

txNew.vout.insert(txNew.vout.begin() + nChangePosInOut, newTxOut);
    } else {
        nChangePosInOut = -1;
    }
    
    std::vector<std::shared_ptr<COutput>> selected_coins = result.GetShuffledInputVector();
    
    
    
    
    
    
    
    
    const uint32_t nSequence{coin_control.m_signal_bip125_rbf.value_or(wallet.m_signal_rbf) ? MAX_BIP125_RBF_SEQUENCE : CTxIn::MAX_SEQUENCE_NONFINAL};
    for (const auto& coin : selected_coins) {
        txNew.vin.push_back(CTxIn(coin->outpoint, CScript(), nSequence));
    }
    DiscourageFeeSniping(txNew, rng_fast, wallet.chain(), wallet.GetLastBlockHash(), wallet.GetLastBlockHeight());
    
    TxSize tx_sizes = CalculateMaximumSignedTxSize(CTransaction(txNew), &wallet, &coin_control);
    int nBytes = tx_sizes.vsize;
    if (nBytes == -1) {
        return util::Error{_("Missing solving data for estimating transaction size")};
    }
    CAmount fee_needed = coin_selection_params.m_effective_feerate.GetFee(nBytes);
    const CAmount output_value = CalculateOutputValue(txNew);
    Assume(recipients_sum + change_amount == output_value);
    CAmount current_fee = result.GetSelectedValue() - output_value;