tids should not return server block on error

[Sam Hartman]

In the traces in https://bugs.launchpad.net/moonshot-tr/+bug/1464800, we see that even in an error situation, the tids is returning server blocks. That's almost certainly wrong.
Also, I wonder whether the intermediate trust routers should trim out server blocks on error responses to avoid a covert-channel-like-thing where we pretend to the middle something failed, but establish a key anyway.