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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

{
    switch (type) {
        case TxoutType::WITNESS_V1_TAPROOT:
            return OutputType::BECH32M;
        case TxoutType::WITNESS_V0_KEYHASH:
        case TxoutType::WITNESS_V0_SCRIPTHASH:
            if (is_from_p2sh) return OutputType::P2SH_SEGWIT;
            else return OutputType::BECH32;
        case TxoutType::SCRIPTHASH:
        case TxoutType::PUBKEYHASH:
            return OutputType::LEGACY;
        default:
            return OutputType::UNKNOWN;
    }
}
util::Result<PreSelectedInputs> FetchSelectedInputs(const CWallet& wallet, const CCoinControl& coin_control,
                                            const CoinSelectionParams& coin_selection_params) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet)
{
    PreSelectedInputs result;
    std::vector<COutPoint> vPresetInputs;
    coin_control.ListSelected(vPresetInputs);
    const bool can_grind_r = wallet.CanGrindR();
    for (const COutPoint& outpoint : vPresetInputs) {
        int input_bytes = -1;
        CTxOut txout;