tests: make run-tests fail early if no channel is found I hit a weird corner case in run-tests where a test that caused an exception to be raised was breaking everything with an unbound variable error a few lines down because channel was never getting set in this for loop. By adding an `else` clause to this for loop, we can explode right away if we can't find a channel and give the developer a better chance at figuring out what's going on.

dispatch: convert exception payload to bytes more carefully We were previously depending on str() doing something reasonable here, and we can't depend on the objects in question supporting __bytes__, so we work around the lack of direct bytes formatting.

help: convert flag default to bytes portably We were relying on %s using repr on (for example) integer values. Work around that for Python 3 while preserving all the prior magic.