Throw a useful error when deserializing an embedded relationship

I had my Serializer misconfigured for an embedded relationship (typo in the relationship name in the attrs hash) and I was greeted with this error.

![screen shot 2014-11-23 at 6 55 15 pm](https://cloud.githubusercontent.com/assets/54056/5159816/769a1764-7342-11e4-8250-47ce9e69d4e7.png)

This pr improves the error message to make the reason for failing more apparent. 

I'm open to any feedback or improvements on the message content.


#### After
![screen shot 2014-11-23 at 6 55 34 pm](https://cloud.githubusercontent.com/assets/54056/5159834/1a85cf76-7343-11e4-9e57-728a7cfa551e.png)