**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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).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 CoinsResult& coins,
                          const CoinSelectionParams& coin_sel_params,
                          const std::vector<SelectionFilter>& filters)
{
    FilteredOutputGroups filtered_groups;
    if (!coin_sel_params.m_avoid_partial_spends) {
        
        for (const auto& [type, outputs] : coins.coins) {
            for (const COutput& output : outputs) {
                
                size_t ancestors, descendants;
                wallet.chain().getTransactionAncestry(output.outpoint.hash, ancestors, descendants);
                
                OutputGroup group(coin_sel_params);
                group.Insert(std::make_shared<COutput>(output), ancestors, descendants);
                
                for (const auto& sel_filter : filters) {
                    const auto& filter = sel_filter.filter;
                    if (!group.EligibleForSpending(filter)) continue;
                    filtered_groups[filter].Push(group, type, true, true);
                }
            }
        }
        return filtered_groups;
    }