Better error message for missing attribute

When an attribute doesn't exist on a model, 

`it { should normalize_attribute(:foo) }`

gives the error:

```
Failure/Error: it { should normalize_attribute(:foo) }
     NoMethodError:
       undefined method `old_foo=' for #<Bar:0x007fe72c21f2a8>
```

It seems to me this error message should be more useful by telling you the attribute is missing.
