CONT_SET_EXC: Set exception on the continuation, to be rethrown
by the caller.
CONT_SET_EXC_AND_RET: Convenience which returns immediately
after setting the exception.
Implements: LockSet, SerializedAsynchronousContinuation,
LockerAndInvoker, LockerAndInvokerBase, Qutex.
Very big leap in functionality here. See qutexes.md for
an explanation of what we've done.
hayodea