Do not send a not-connected error when we receive an unavailable presence
When we receive a stanza with a “to” targetting an IRC server to which biboumi is not connected, we send an error stanza to signal that. But we shouldn’t send that error in the case of . These presences are often used before a join presence, to make sure the server really nows that we want a full join. Biboumi shouldn’t care whether or not we’re connected to the channel: if we’re not, do nothing (no error), and if we are, part from the channel.