**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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).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<CTxOut> txouts;
    
    for (const CTxIn& input : tx.vin) {
        const auto mi = wallet->mapWallet.find(input.prevout.hash);
        
        if (mi != wallet->mapWallet.end()) {
            assert(input.prevout.n < mi->second.tx->vout.size());
            txouts.emplace_back(mi->second.tx->vout.at(input.prevout.n));
        } else if (coin_control) {
            CTxOut txout;
            if (!coin_control->GetExternalOutput(input.prevout, txout)) {
                return TxSize{-1, -1};
            }
            txouts.emplace_back(txout);
        } else {
            return TxSize{-1, -1};
        }
    }
    return CalculateMaximumSignedTxSize(tx, wallet, txouts, coin_control);
}
size_t CoinsResult::Size() const