@joeyAghion => Improve error handling in ow:console.

Otherwise this:

```
rake aborted!
undefined method `[]' for nil:NilClass
/Users/dblock/.rvm/gems/ruby-2.0.0-p353/gems/momentum-0.0.9/lib/momentum/opsworks.rb:21:in `get_instance_endpoint'
/Users/dblock/.rvm/gems/ruby-2.0.0-p353/gems/momentum-0.0.9/lib/tasks/ow-console.rake:14:in `block (2 levels) in <top (required)>'
/Users/dblock/.rvm/gems/ruby-2.0.0-p353/bin/ruby_executable_hooks:15:in `eval'
/Users/dblock/.rvm/gems/ruby-2.0.0-p353/bin/ruby_executable_hooks:15:in `<main>'
Tasks: TOP => ow:console
```