[pinpoint] Separate Execution exceptions from result_values.

dashboard/dashboard/pinpoint/models/attempt.py

Index: dashboard/dashboard/pinpoint/models/attempt.py
diff --git a/dashboard/dashboard/pinpoint/models/attempt.py b/dashboard/dashboard/pinpoint/models/attempt.py
index 3fc153ff6067110e4bd436ba44e1a9e515c62fd7..0ecb02e90d80ccf358e1ceb8cc11756893ee32d9 100644
--- a/dashboard/dashboard/pinpoint/models/attempt.py
+++ b/dashboard/dashboard/pinpoint/models/attempt.py
@@ -42,6 +42,11 @@ class Attempt(object):
         self._last_execution.completed and
         len(self._quests) == len(self._executions))
 
+  @property
+  def exception(self):
+    assert self.completed
+    return self._last_execution.exception
+
   @property
   def result_values(self):
     assert self.completed