Try to call #constantize on errors_controller

Sometimes we want to call this code:

```ruby
Gaffe.configure do |config|
  config.errors_controller = ErrorsController
end

Gaffe.enable!
```

But starting Rails might fail because auto-loading `ErrorsController`  this soon could raise exceptions because other initializers might be needed first.

So we can now to this:

```ruby
Gaffe.configure do |config|
  config.errors_controller = 'ErrorsController'
end

Gaffe.enable!
```

And `"ErrorsController"` will be converted to its constant when we need it.