Review: fix argument error when raising this exception

Surprise:

```ruby
irb(main):002:0> raise Rets::AuthorizationFailure, 1, 2
ArgumentError: wrong number of arguments (1 for 2)
  from /Users/tcrayford/Projects/estately/rets/lib/rets.rb:14:in
`initialize'
  from (irb):2:in `exception'
  from (irb):2:in `raise'
  from (irb):2
  from /Users/tcrayford/.rbenv/versions/1.9.3-p0/bin/irb:12:in `<main>'
```