Partial blocks don't failover when partial doesn't exist

Partial calls with a failover should [render the failover content](http://handlebarsjs.com/partials.html#partial-block) when the partial doesn't exist. As is it will instead give the error `Error: Could not locate a partial named myPartial`.

e.g. this should be valid standing alone 
```hbs
{{#> myPartial }}
  Failover content
{{/myPartial}}
```
