Fixed handling of missing ACK in Session

Ignore-this: edaadfad6feb58fee99ce47bae87a89d

darcs-hash:20100429102740-1e3d8-63433f91a1367d54854c05f35518d67940ab2388
Improved error message

darcs-hash:20100429105345-58252-a477f4711fdae2b829921b9a5ce833d2bc4a0ec4