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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

push_output_groups(spk_to_positive_groups_map,  true);
    return filtered_groups;
}
static bool HasErrorMsg(const util::Result<SelectionResult>& res) { return !util::ErrorString(res).empty(); }
util::Result<SelectionResult> AttemptSelection(const CAmount& nTargetValue, OutputGroupTypeMap& groups,
                               const CoinSelectionParams& coin_selection_params, bool allow_mixed_output_types)
{
    
    std::vector<SelectionResult> results;
    for (auto& [type, group] : groups.groups_by_type) {
        auto result{ChooseSelectionResult(nTargetValue, group, coin_selection_params)};
        
        if (HasErrorMsg(result)) return result; 
        
        if (result) results.push_back(*result);
    }
    
    
    if (results.size() > 0) return *std::min_element(results.begin(), results.end());
    
    
    
    if (allow_mixed_output_types && groups.TypesCount() > 1) {
        return ChooseSelectionResult(nTargetValue, groups.all_groups, coin_selection_params);
    }
    
    
    return util::Error();
};
util::Result<SelectionResult> ChooseSelectionResult(const CAmount& nTargetValue, Groups& groups, const CoinSelectionParams& coin_selection_params)