Be more careful around IOExceptions.

This adds a first test where a frame is truncated. That has some
unpleasant consequences for the other tests because it means the
MockSpdyPeer is more aggressive about closing the socket when all
frames have been sent.

It also reduces the use of exceptions for flow control when handling
bogus incoming frames. This is a bit worse and a bit better; my
goal is to make it easier to differentiate between protocol-level
problems (bogus frames) from transport-level problems (closed sockets
and EOF streams).