Better error handling when searching for policy_class method

In Ruby 1.9.3 Object#respond_to? can return true even if the method is not defined on the public interface of the object. This can happen if for some reason the policy_class method was protected. This PR should change the error being raised from:


    NoMethodError - undefined method `policy_class' for #<Class:0x00000009a55f38>
    ...
    pundit (0.2.3) lib/pundit/policy_finder.rb:35:in `find'
    pundit (0.2.3) lib/pundit/policy_finder.rb:28:in `policy!'
    ...

to the more useful error:

    Pundit::NotDefinedError - unable to find policy SomePolicy