Adding missing error class

src/main/java/de/bitsharesmunich/graphenej/AssetAmount.java

@@ -3,6 +3,7 @@ package de.bitsharesmunich.graphenej;
 import com.google.common.primitives.Bytes;
 import com.google.common.primitives.UnsignedLong;
 import com.google.gson.*;
+import de.bitsharesmunich.graphenej.errors.IncompatibleOperation;
 import de.bitsharesmunich.graphenej.interfaces.ByteSerializable;
 import de.bitsharesmunich.graphenej.interfaces.JsonSerializable;
 

src/main/java/de/bitsharesmunich/graphenej/api/GetAllAssetHolders.java

@@ -0,0 +1,7 @@
+package de.bitsharesmunich.graphenej.api;
+
+/**
+ * Created by nelson on 1/25/17.
+ */
+public class GetAllAssetHolders {
+}

src/main/java/de/bitsharesmunich/graphenej/errors/IncompatibleOperation.java

@@ -0,0 +1,11 @@
+package de.bitsharesmunich.graphenej.errors;
+
+/**
+ * Created by nelson on 1/18/17.
+ */
+public class IncompatibleOperation extends RuntimeException {
+
+    public IncompatibleOperation(String message){
+        super(message);
+    }
+}

src/main/java/de/bitsharesmunich/graphenej/models/HoldersCount.java

@@ -0,0 +1,7 @@
+package de.bitsharesmunich.graphenej.models;
+
+/**
+ * Created by nelson on 1/25/17.
+ */
+public class HoldersCount {
+}