**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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).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 (IsCurrentForAntiFeeSniping(chain, block_hash)) {
        tx.nLockTime = block_height;
        
        
        
        
        if (rng_fast.randrange(10) == 0) {
            tx.nLockTime = std::max(0, int(tx.nLockTime) - int(rng_fast.randrange(100)));
        }
    } else {
        
        
        
        tx.nLockTime = 0;
    }
    
    assert(tx.nLockTime < LOCKTIME_THRESHOLD); 
    assert(tx.nLockTime <= uint64_t(block_height));
    for (const auto& in : tx.vin) {
        
        assert(in.nSequence != CTxIn::SEQUENCE_FINAL);
        
        if (in.nSequence == CTxIn::MAX_SEQUENCE_NONFINAL) continue;
        
        if (in.nSequence == MAX_BIP125_RBF_SEQUENCE) continue;