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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

PreSelectedInputs preset_inputs;
    if (coin_control.HasSelected()) {
        auto res_fetch_inputs = FetchSelectedInputs(wallet, coin_control, coin_selection_params);
        if (!res_fetch_inputs) return util::Error{util::ErrorString(res_fetch_inputs)};
        preset_inputs = *res_fetch_inputs;
    }
    
    
    CoinsResult available_coins;
    if (coin_control.m_allow_other_inputs) {
        available_coins = AvailableCoins(wallet, &coin_control, coin_selection_params.m_effective_feerate);
    }
    
    auto select_coins_res = SelectCoins(wallet, available_coins, preset_inputs, selection_target, coin_control, coin_selection_params);
    if (!select_coins_res) {
        
        const bilingual_str& err = util::ErrorString(select_coins_res);
        return util::Error{err.empty() ?_("Insufficient funds") : err};
    }
    const SelectionResult& result = *select_coins_res;
    TRACE5(coin_selection, selected_coins, wallet.GetName().c_str(), GetAlgorithmName(result.GetAlgo()).c_str(), result.GetTarget(), result.GetWaste(), result.GetSelectedValue());
    const CAmount change_amount = result.GetChange(coin_selection_params.min_viable_change, coin_selection_params.m_change_fee);
    if (change_amount > 0) {
        CTxOut newTxOut(change_amount, scriptChange);
        if (nChangePosInOut == -1) {
            
            nChangePosInOut = rng_fast.randrange(txNew.vout.size() + 1);
        } else if ((unsigned int)nChangePosInOut > txNew.vout.size()) {
            return util::Error{_("Transaction change output index out of range")};
        }