Flag unknown collections in joins @ compile-time

[neilconway]

If a join references a nonexistent collection, we get a runtime exception like so:

Exception: #<NameError: undefined local variable or method `xxxnode_count' for #<RingAft:0x101cc8f60>>.
Rule: aft_send <= ((token_persist * next_node).pairs do |tp, nn|
  [((@node_id + 1) % xxxnode_count[[]].num), 0]
end)>

This should be fairly easily catchable at compile-time.