oft: handle missing test module/class docstrings
action_list: throw exception if value is not an action

This is a programmer error, not a switch issue.
throw AssertionError if controller.message_send fails

Unlike other types of exception an AssertionError will cause the test to "fail"
instead of "error". Using an exception instead of checking return values
removes a lot of boilerplate code that wasn't always present (or correct). It
may also help disabuse test writers of the dangerous notion that succeeding in
writing a message to the TCP socket means anything.
remove assertions on the return value of delete_all_flows

This function can no longer fail.
raise an AssertionError if do_barrier fails

This removes a lot of boilerplate code.
remove unused oft_assert module