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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

results.push_back(*srd_result);
    }
    if (results.empty()) {
        
        return util::Error();
    }
    std::vector<SelectionResult> eligible_results;
    std::copy_if(results.begin(), results.end(), std::back_inserter(eligible_results), [coin_selection_params](const SelectionResult& result) {
        const auto initWeight{coin_selection_params.tx_noinputs_size * WITNESS_SCALE_FACTOR};
        return initWeight + result.GetWeight() <= static_cast<int>(MAX_STANDARD_TX_WEIGHT);
    });
    if (eligible_results.empty()) {
        return util::Error{_("The inputs size exceeds the maximum weight. "
                             "Please try sending a smaller amount or manually consolidating your wallet's UTXOs")};
    }
    
    
    auto& best_result = *std::min_element(eligible_results.begin(), eligible_results.end());
    return best_result;
}
util::Result<SelectionResult> SelectCoins(const CWallet& wallet, CoinsResult& available_coins, const PreSelectedInputs& pre_set_inputs,
                                          const CAmount& nTargetValue, const CCoinControl& coin_control,
                                          const CoinSelectionParams& coin_selection_params)
{
    
    CAmount selection_target = nTargetValue - pre_set_inputs.total_amount;
    
    if (!coin_control.m_allow_other_inputs && selection_target > 0) {
        return util::Error{_("The preselected coins total amount does not cover the transaction target. "
                             "Please allow other inputs to be automatically selected or include more coins manually")};