#598 - throw exceptions on unexpected method calls

src/main/java/de/deadlocker8/budgetmaster/database/model/v4/BackupDatabase_v4.java

@@ -108,6 +108,6 @@ public class BackupDatabase_v4 implements BackupDatabase
 	@Override
 	public InternalDatabase convertToInternal()
 	{
-		return null;
+		throw new UnsupportedOperationException();
 	}
 }

src/main/java/de/deadlocker8/budgetmaster/database/model/v5/BackupDatabase_v5.java

@@ -136,7 +136,7 @@ public class BackupDatabase_v5 implements BackupDatabase
 	@Override
 	public BackupDatabase upgrade()
 	{
-		return null;
+		throw new UnsupportedOperationException();
 	}
 
 	public static BackupDatabase_v5 createFromInternalEntities(InternalDatabase database)