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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

CWallet& wallet,
        const std::vector<CRecipient>& vecSend,
        int change_pos,
        const CCoinControl& coin_control,
        bool sign)
{
    if (vecSend.empty()) {
        return util::Error{_("Transaction must have at least one recipient")};
    }
    if (std::any_of(vecSend.cbegin(), vecSend.cend(), [](const auto& recipient){ return recipient.nAmount < 0; })) {
        return util::Error{_("Transaction amounts must not be negative")};
    }
    LOCK(wallet.cs_wallet);