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  The discussion in https://github.com/bitcoin/bitcoin/pull/10015 is relevant (it appears that the PR was withdrawn for reasons unrelated to my request).

}
    
    CScript scriptChange;
    bilingual_str error; 
    
    if (!std::get_if<CNoDestination>(&coin_control.destChange)) {
        scriptChange = GetScriptForDestination(coin_control.destChange);
    } else { 
        
        
        
        
        
        
        
        
        CTxDestination dest;
        auto op_dest = reservedest.GetReservedDestination(true);
        if (!op_dest) {
            error = _("Transaction needs a change address, but we can't generate it.") + Untranslated(" ") + util::ErrorString(op_dest);
        } else {
            dest = *op_dest;
            scriptChange = GetScriptForDestination(dest);
        }
        
        
        
        CHECK_NONFATAL(IsValidDestination(dest) != scriptChange.empty());
    }
    CTxOut change_prototype_txout(0, scriptChange);