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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

typedef std::map<std::pair<CScript, OutputType>, std::vector<OutputGroup>> ScriptPubKeyToOutgroup;
    const auto& insert_output = [&](
            const std::shared_ptr<COutput>& output, OutputType type, size_t ancestors, size_t descendants,
            ScriptPubKeyToOutgroup& groups_map) {
        std::vector<OutputGroup>& groups = groups_map[std::make_pair(output->txout.scriptPubKey,type)];
        if (groups.size() == 0) {
            
            groups.emplace_back(coin_sel_params);
        }
        
        
        OutputGroup* group = &groups.back();
        
        
        if (group->m_outputs.size() >= OUTPUT_GROUP_MAX_ENTRIES) {
            
            groups.emplace_back(coin_sel_params);
            group = &groups.back();
        }
        group->Insert(output, ancestors, descendants);
    };
    ScriptPubKeyToOutgroup spk_to_groups_map;
    ScriptPubKeyToOutgroup spk_to_positive_groups_map;
    for (const auto& [type, outs] : coins.coins) {
        for (const COutput& output : outs) {
            size_t ancestors, descendants;
            wallet.chain().getTransactionAncestry(output.outpoint.hash, ancestors, descendants);
            const auto& shared_output = std::make_shared<COutput>(output);