Handle limits better

In #389 (one) core problem turned out to be with Twisted's chosen limits in LineOnlyReceiver: if "too much" is buffered, the connection is closed.

We should provide a better error than this for txtorcon users so that it's more obvious in the future that limits may need to be increased again.

(A longer-term solution would be to determine what Tor's actual limits -- if any -- are here, and use those).