fix session2: listening to wrong exception caused spurious ValueError tracebacks printed

darcs-hash:20090430070407-b2873-f9921f4e1fa40613e0b8bec979df93399b2a6f57
session2: don't print the traceback for ValueError when removing a stream

darcs-hash:20090430070504-b2873-419c87c85bc9cb4694ab00918dc9306a5356a25e