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
I prefer the `if/else` spelling, but if we're going to use `or`...

``` ruby
associated_class._reflect_on_association(options[:inverse_of]) or
  raise InverseOfAssociationNotFoundError.new(self, associated_class)
```