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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

assert(false);
    }
}
static util::Result<CreatedTransactionResult> CreateTransactionInternal(
        CWallet& wallet,
        const std::vector<CRecipient>& vecSend,
        int change_pos,
        const CCoinControl& coin_control,
        bool sign) EXCLUSIVE_LOCKS_REQUIRED(wallet.cs_wallet)
{
    AssertLockHeld(wallet.cs_wallet);
    
    int nChangePosInOut = change_pos;
    FastRandomContext rng_fast;
    CMutableTransaction txNew; 
    CoinSelectionParams coin_selection_params{rng_fast}; 
    coin_selection_params.m_avoid_partial_spends = coin_control.m_avoid_partial_spends;
    coin_selection_params.m_include_unsafe_inputs = coin_control.m_include_unsafe_inputs;
    
    coin_selection_params.m_long_term_feerate = wallet.m_consolidate_feerate;
    CAmount recipients_sum = 0;
    const OutputType change_type = wallet.TransactionChangeType(coin_control.m_change_type ? *coin_control.m_change_type : wallet.m_default_change_type, vecSend);
    ReserveDestination reservedest(&wallet, change_type);
    unsigned int outputs_to_subtract_fee_from = 0; 
    for (const auto& recipient : vecSend) {
        recipients_sum += recipient.nAmount;
        if (recipient.fSubtractFeeFromAmount) {
            outputs_to_subtract_fee_from++;
            coin_selection_params.m_subtract_fee_outputs = true;
        }