added predecessor_status field now returns 'no_predecessor' if there is no predecessor

atdb/taskdatabase/models.py

@@ -62,7 +62,7 @@ class Task(models.Model):
         try:
             return self.predecessor.status
         except:
-            return "unknown"
+            return "no_predecessor"
 
 class LogEntry(models.Model):
     cpu_cycles = models.IntegerField(null=True,blank=True)