Bug 1490741 - Don't retry tasks that fail with IndexError (#4365)

Since it's unlikely to be a recoverable error.

treeherder/workers/task.py

@@ -15,6 +15,7 @@ class retryable_task(object):
     """Wrapper around a celery task to add conditional task retrying."""
 
     NON_RETRYABLE_EXCEPTIONS = (
+        IndexError,
         IntegrityError,
         jsonschema.ValidationError,
         KeyError,