Add the "failure?" method to the outcome

The failure? method helps consumers of the mutations add some clarity.

Sometimes after a mutation, you really just want to recover gracefully on failure and I've found myself with plenty of code resembling the following:

```ruby
mutation = SomeMutationClass.run(some_params)
unless mutation.success?
  # do something to recover, like attempt something else
end
```

Having the `failure?` method makes things just a little clearer since the code can now ask `if failure?`.