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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

const CCoinControl* coinControl,
                           std::optional<CFeeRate> feerate,
                           const CoinFilterParams& params)
{
    AssertLockHeld(wallet.cs_wallet);
    CoinsResult result;
    
    
    bool allow_used_addresses = !wallet.IsWalletFlagSet(WALLET_FLAG_AVOID_REUSE) || (coinControl && !coinControl->m_avoid_address_reuse);
    const int min_depth = {coinControl ? coinControl->m_min_depth : DEFAULT_MIN_DEPTH};
    const int max_depth = {coinControl ? coinControl->m_max_depth : DEFAULT_MAX_DEPTH};
    const bool only_safe = {coinControl ? !coinControl->m_include_unsafe_inputs : true};
    const bool can_grind_r = wallet.CanGrindR();
    std::set<uint256> trusted_parents;
    for (const auto& entry : wallet.mapWallet)
    {
        const uint256& wtxid = entry.first;
        const CWalletTx& wtx = entry.second;
        if (wallet.IsTxImmatureCoinBase(wtx) && !params.include_immature_coinbase)
            continue;
        int nDepth = wallet.GetTxDepthInMainChain(wtx);
        if (nDepth < 0)
            continue;
        
        
        if (nDepth == 0 && !wtx.InMempool())
            continue;
        bool safeTx = CachedTxIsTrusted(wallet, wtx, trusted_parents);