Reduce complexity in 2  _inverse_of methods

In polymoprhic_inverse_of:
Use an early return and a guard to raise an error instead of nested if statements which in the true/true case returns the condition of the if statement

In automatic_inverse_of:
Fix the comment to say false instead of nil
Reduce complexity by moving to a early return and a method level rescue instead of a begin..rescue..end.
Eliminates an explicit return in the middle of the method
We don't need to talk about the `NameError` case here. In fact the before text were fine:

```ruby
# Returns either false or the inverse association name that it finds.
```

If it doesn't find so false is returned.