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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

}
    
    if (selection_target <= 0) {
        SelectionResult result(nTargetValue, SelectionAlgorithm::MANUAL);
        result.AddInputs(pre_set_inputs.coins, coin_selection_params.m_subtract_fee_outputs);
        result.ComputeAndSetWaste(coin_selection_params.min_viable_change, coin_selection_params.m_cost_of_change, coin_selection_params.m_change_fee);
        return result;
    }
    
    
    CAmount available_coins_total_amount = coin_selection_params.m_subtract_fee_outputs ? available_coins.GetTotalAmount() :
            (available_coins.GetEffectiveTotalAmount().has_value() ? *available_coins.GetEffectiveTotalAmount() : 0);
    if (selection_target > available_coins_total_amount) {
        return util::Error(); 
    }
    
    auto op_selection_result = AutomaticCoinSelection(wallet, available_coins, selection_target, coin_selection_params);
    if (!op_selection_result) return op_selection_result;
    
    if (!pre_set_inputs.coins.empty()) {
        SelectionResult preselected(pre_set_inputs.total_amount, SelectionAlgorithm::MANUAL);
        preselected.AddInputs(pre_set_inputs.coins, coin_selection_params.m_subtract_fee_outputs);
        op_selection_result->Merge(preselected);
        op_selection_result->ComputeAndSetWaste(coin_selection_params.min_viable_change,
                                                coin_selection_params.m_cost_of_change,
                                                coin_selection_params.m_change_fee);
    }
    return op_selection_result;
}
util::Result<SelectionResult> AutomaticCoinSelection(const CWallet& wallet, CoinsResult& available_coins, const CAmount& value_to_select, const CoinSelectionParams& coin_selection_params)