fix removecoin method

[pbca26]

If it's not possible to make a complete coin removal could you make Iguana stop responding to any api calls? Right now it's still responding back after I executed removecoin method.

curl --url "http://127.0.0.1:7778" --data "{"agent":"iguana","method":"removecoin","activecoin":"SYS"}"

{"result":"success","tag":"7764525998497036149"}

curl --url "http://127.0.0.1:7778" --data "{"agent":"bitcoinrpc","method":"getinfo"}"

{"result":"success","protocolversion":70003,"kbfee":0.00001250,"txfee":0.00010000,"bundles":99.17355372,"utxo":99.17355372,"balances":99.17355372,"validated":99.17355372,"firstRTheight":240000,"RTheight":241837,"blocks":242000,"longestchain":242913,"port":8369,"supernet":[],"connections":13,"difficulty":241723.00068090,"status":"SYS.RT241837 u.120+c.120 b.120 v.120 (0+58/2000 1st.120).s0 to 121 N[122] h.242001 r.240000 c.240000 s.240058 d.0 E.120 maxB.18 peers.13/256 Q.(21429 0) (L.242913 121:913) M.242000 3d36c0559c8f19269a8080cd28d93875959c62813ef7fafe43fa0837f9216ec3 ledger.a9e7c7651ba1fdd2 supply 4277213804.38371277 sigs 0:0","coin":"SYS","tag":"1150455986746551596"}

[jl777]

dPoW branch returns error for requests to removed coin