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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

ordered_filters.push_back({CoinEligibilityFilter(0, 1, max_ancestors-1, max_descendants-1, true)});
            
            
            if (coin_selection_params.m_include_unsafe_inputs) {
                ordered_filters.push_back({CoinEligibilityFilter(0, 0, max_ancestors-1, max_descendants-1, true)});
            }
            
            
            
            if (!fRejectLongChains) {
                ordered_filters.push_back({CoinEligibilityFilter(0, 1, std::numeric_limits<uint64_t>::max(),
                                                                   std::numeric_limits<uint64_t>::max(),
                                                                   true)});
            }
        }
        
        FilteredOutputGroups filtered_groups = GroupOutputs(wallet, available_coins, coin_selection_params, ordered_filters);
        
        
        
        std::vector<util::Result<SelectionResult>> res_detailed_errors;
        for (const auto& select_filter : ordered_filters) {
            auto it = filtered_groups.find(select_filter.filter);
            if (it == filtered_groups.end()) continue;
            if (auto res{AttemptSelection(value_to_select, it->second,
                                          coin_selection_params, select_filter.allow_mixed_output_types)}) {
                return res;