Join predicate error checking

[neilconway]

Here's a buggy statement:

    msg_recv <~ (new_msg * node_status * attempt_status).combos(msg_send.recv_node => node_status.node_id, node_status.attempt_id => attempt_status.attempt_id) do |m, ns, as|
      [as.addr, m.recv_id, m.recv_node, m.send_node, m.payload] if as.status == ATTEMPT_LIVE
    end

The bug is that I had replaced msg_send with new_msg in the join constructor (starred list), but not in the join conditions. Bud silently executes this code, but it produces weird outputs (I'm guessing because it is projecting against tuples with an unexpected schema).

Expected behavior: Bud should flag this error, either at syntax-check time (preferrable) or at run-time (good enough).