Fix exception raising only once for a given ThreadWithException.

* bzrlib/tests/test_test_server.py:
(TestTCPServerInAThread): Check that we can caught an exception
raised while serving.

* bzrlib/tests/test_server.py:
(ThreadWithException.join): Fix typo.
(TestingTCPServer._pending_exception)
(TestingThreadingTCPServer._pending_exception): Check pending
exception in involved threads.
(TestingTCPServerInAThread.pending_exception): Forward exception
check to server and its optional threads.