Don't mark wallet as dirty if key already exists.

Fix incorrect method name used in error message.

src/wallet/rpcdump.cpp

@@ -565,12 +565,12 @@ Value z_importkey(const Array& params, bool fHelp)
     auto addr = key.address();
 
     {
-        pwalletMain->MarkDirty();
-
         // Don't throw error in case a key is already there
         if (pwalletMain->HaveSpendingKey(addr))
             return Value::null;
 
+        pwalletMain->MarkDirty();
+
         if (!pwalletMain-> AddZKey(key))
             throw JSONRPCError(RPC_WALLET_ERROR, "Error adding spending key to wallet");