**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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).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 (output.GetEffectiveValue() > 0) {
                insert_output(shared_output, type, ancestors, descendants, spk_to_positive_groups_map);
            }
            
            insert_output(shared_output, type, ancestors, descendants, spk_to_groups_map);
        }
    }
    
    const auto& push_output_groups = [&](const ScriptPubKeyToOutgroup& groups_map, bool positive_only) {
        for (const auto& [script, groups] : groups_map) {
            
            for (auto group_it = groups.rbegin(); group_it != groups.rend(); group_it++) {
                const OutputGroup& group = *group_it;
                
                for (const auto& sel_filter : filters) {
                    const auto& filter = sel_filter.filter;
                    if (!group.EligibleForSpending(filter)) continue;
                    
                    if (group_it == groups.rbegin() && groups.size() > 1 && !filter.m_include_partial_groups) {
                        continue;
                    }
                    OutputType type = script.second;
                    
                    filtered_groups[filter].Push(group, type, positive_only, !positive_only);
                }
            }
        }
    };
    push_output_groups(spk_to_groups_map,  false);