"not connected" is now a checked Exception thrown by sendPacket()

There is a unsolveable race condition between the connection state and
sendPacket(), i.e. the connection could go down, right after the
method calling sendPacket is called, but before sendPacket() is
invoked. Before this change, sendPacket() has thrown an unchecked
IllegalStateException, which could be ignored by the Smack user, who
would also not notice the race condition. We have decided to throw a
checked Exception in this case now, to make the Smack user aware of
this situation.