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

Conflicts:
	src/rpcwallet.cpp

src/bitcoinrpc.h

@@ -50,10 +50,13 @@ enum RPCErrorCode
     RPC_INVALID_PARAMETER           = -8,  // Invalid, missing or duplicate parameter
     RPC_DATABASE_ERROR              = -20, // Database error
     RPC_DESERIALIZATION_ERROR       = -22, // Error parsing or validating structure in raw format
+    RPC_SERVER_NOT_STARTED          = -18, // RPC server was not started (StartRPCThreads() not called)
 
     // P2P client errors
     RPC_CLIENT_NOT_CONNECTED        = -9,  // Bitcoin is not connected
     RPC_CLIENT_IN_INITIAL_DOWNLOAD  = -10, // Still downloading initial blocks
+    RPC_CLIENT_NODE_ALREADY_ADDED   = -23, // Node is already added
+    RPC_CLIENT_NODE_NOT_ADDED       = -24, // Node has not been added before
 
     // Wallet errors
     RPC_WALLET_ERROR                = -4,  // Unspecified problem with wallet (key not found etc.)

src/rpcnet.cpp

@@ -120,13 +120,13 @@ Value addnode(const Array& params, bool fHelp)
     if (strCommand == "add")
     {
         if (it != vAddedNodes.end())
-            throw JSONRPCError(-23, "Error: Node already added");
+            throw JSONRPCError(RPC_CLIENT_NODE_ALREADY_ADDED, "Error: Node already added");
         vAddedNodes.push_back(strNode);
     }
     else if(strCommand == "remove")
     {
         if (it == vAddedNodes.end())
-            throw JSONRPCError(-24, "Error: Node has not been added.");
+            throw JSONRPCError(RPC_CLIENT_NODE_NOT_ADDED, "Error: Node has not been added.");
         vAddedNodes.erase(it);
     }
 
@@ -163,7 +163,7 @@ Value getaddednodeinfo(const Array& params, bool fHelp)
                 break;
             }
         if (laddedNodes.size() == 0)
-            throw JSONRPCError(-24, "Error: Node has not been added.");
+            throw JSONRPCError(RPC_CLIENT_NODE_NOT_ADDED, "Error: Node has not been added.");
     }
 
     if (!fDns)

src/rpcwallet.cpp

@@ -1391,6 +1391,8 @@ Value walletpassphrase(const Array& params, bool fHelp)
             "if [stakingonly] is true sending functions are disabled.");
     if (fHelp)
         return true;
+    if (!fServer)
+        throw JSONRPCError(RPC_SERVER_NOT_STARTED, "Error: RPC server was not started, use server=1 to change this.");
     if (!pwalletMain->IsCrypted())
         throw JSONRPCError(RPC_WALLET_WRONG_ENC_STATE, "Error: running with an unencrypted wallet, but walletpassphrase was called.");