**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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).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 (auto ptr_wtx = wallet.GetWalletTx(outpoint.hash)) {
            
            if (ptr_wtx->tx->vout.size() <= outpoint.n) {
                return util::Error{strprintf(_("Invalid pre-selected input %s"), outpoint.ToString())};
            }
            txout = ptr_wtx->tx->vout.at(outpoint.n);
            input_bytes = CalculateMaximumSignedInputSize(txout, &wallet, &coin_control);
        } else {
            
            if (!coin_control.GetExternalOutput(outpoint, txout)) {
                return util::Error{strprintf(_("Not found pre-selected input %s"), outpoint.ToString())};
            }
        }
        if (input_bytes == -1) {
            input_bytes = CalculateMaximumSignedInputSize(txout, outpoint, &coin_control.m_external_provider, can_grind_r, &coin_control);
        }
        
        if (coin_control.HasInputWeight(outpoint)) {
            input_bytes = GetVirtualTransactionSize(coin_control.GetInputWeight(outpoint), 0, 0);
        }
        if (input_bytes == -1) {
            return util::Error{strprintf(_("Not solvable pre-selected input %s"), outpoint.ToString())}; 
        }
        
        COutput output(outpoint, txout,  0, input_bytes,  true,  true,  true,  0,  false, coin_selection_params.m_effective_feerate);
        result.Insert(output, coin_selection_params.m_subtract_fee_outputs);
    }
    return result;
}
CoinsResult AvailableCoins(const CWallet& wallet,