Change assertion to a ValueError in signal handler

letsencrypt/client/standalone_authenticator.py

@@ -60,7 +60,7 @@ class StandaloneAuthenticator(object):
             self.subproc_state = "cantbind"
         else:
             # NOTREACHED
-            assert False
+            raise ValueError("Unexpected signal in signal handler")
 
     def subproc_signal_handler(self, sig, unused_frame):
         """Signal handler for the child process.

letsencrypt/client/tests/standalone_authenticator_test.py

@@ -123,7 +123,7 @@ class ClientSignalHandlerTest(unittest.TestCase):
         # function is only set as a signal handler for the above three
         # signals).
         self.assertRaises(
-            AssertionError, self.authenticator.client_signal_handler,
+            ValueError, self.authenticator.client_signal_handler,
             signal.SIGPIPE, None)