Improve failure message when `has_many` is missing

When test driving, writing an expectation such as

```ruby
it { should have_many(:children).through(:relationships).source(:child) }
```

used to provide the not-so-helpful error message

```
Failure/Error: it { should have_many(:children).through(:relationships).source(:child) }

     NoMethodError:

       undefined method `options' for nil:NilClass
```

The improved error message is

```
Expected Parent to have a has_many association called children (no association called children)
```