RPC: error code changes and prevent crash with walletpassphrase
- fix crash with walletpassphrase by checking if RPC server is running and give a friendly error message how to fix this (fixes #3100)
- add 3 new RPCErrorCodes RPC_SERVER_NOT_STARTED, RPC_NODE_ALREADY_ADDED and RCP_NODE_NOT_ADDED (I checked the source to not use a number already in use for RPC_SERVER_NOT_STARTED)
- use the new codes where needed / missing
- add missing use of RPC_INVALID_PARAMETER