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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

std::vector<std::vector<uint8_t>> script_solutions;
            TxoutType type = Solver(output.scriptPubKey, script_solutions);
            
            
            
            
            bool is_from_p2sh{false};
            if (type == TxoutType::SCRIPTHASH && solvable) {
                CScript script;
                if (!provider->GetCScript(CScriptID(uint160(script_solutions[0])), script)) continue;
                type = Solver(script, script_solutions);
                is_from_p2sh = true;
            }
            result.Add(GetOutputType(type, is_from_p2sh),
                       COutput(outpoint, output, nDepth, input_bytes, spendable, solvable, safeTx, wtx.GetTxTime(), tx_from_me, feerate));
            
            if (params.min_sum_amount != MAX_MONEY) {
                if (result.GetTotalAmount() >= params.min_sum_amount) {
                    return result;
                }
            }
            
            if (params.max_count > 0 && result.Size() >= params.max_count) {
                return result;
            }
        }
    }
    return result;
}
CoinsResult AvailableCoinsListUnspent(const CWallet& wallet, const CCoinControl* coinControl, CoinFilterParams params)