readwrite: add missing traceback object

An error code path was missing the traceback
object and was causing a key error when
that path was executed.

Signed-off-by: Joe Buck <jbbuck@gmail.com>

s3tests/readwrite.py

@@ -53,6 +53,7 @@ def reader(bucket, worker_id, file_names, queue, rand):
                 result.update(
                     error=dict(
                         msg='md5sum check failed',
+                        traceback=traceback.format_exc(),
                         ),
                     )
             else: