Better support for passing around TaskException as pickled value.

Fixes #536

huey/exceptions.py

@@ -5,10 +5,10 @@ class TaskLockedException(HueyException): pass
 class CancelExecution(Exception): pass
 class RetryTask(Exception): pass
 class TaskException(Exception):
-    def __init__(self, metadata, *args):
-        self.metadata = metadata
+    def __init__(self, metadata=None, *args):
+        self.metadata = metadata or {}
         super(TaskException, self).__init__(*args)
 
     def __unicode__(self):
-        return self.metadata['error']
+        return self.metadata.get('error') or 'unknown error'
     __str__ = __unicode__

huey/tests/test_api.py

@@ -433,6 +433,22 @@ def task_e(n):
         self.assertEqual(self.huey.result_count(), 0)
         self.assertEqual(len(self.huey), 0)
 
+    def test_subtask_error(self):
+        @self.huey.task()
+        def task_i(a):
+            raise TestError(a)
+
+        @self.huey.task()
+        def task_o(a):
+            res = task_i(a)
+            self.execute_next()
+            return res.get(blocking=True)
+
+        r = task_o(1)
+        self.assertTrue(self.execute_next() is None)
+        exc = self.trap_exception(r)
+        self.assertEqual(exc.metadata['error'], 'TaskException()')
+
     def test_retry(self):
         class TestException(Exception):
             counter = 0