test: Print traceback when backup restoration unexpectedly fails.

To ease debugging.

caucase/test.py

@@ -35,6 +35,7 @@ import sys
 import tempfile
 import threading
 import time
+import traceback
 import unittest
 import urlparse
 from cryptography import x509
@@ -1566,6 +1567,7 @@ class CaucaseTest(unittest.TestCase):
         user2_new_key_path,
       ),
       0,
+      traceback.format_exc(),
     )
 
     after_restore = list(SQLite3Storage(