[Rackspace] Not Found Exceptions

This pull request is an effort to address issue #1739 where some users are finding that the new auth 2.0 implementation defaults them to the dfw region whereas the previous auth 1.0 defaulted some users to the ord region. This pull request updates the NotFound exception to include the region to make it easier to spot the problem.

In implementing this, I found out that the Rackspace provider was inconsistently namespacing NotFound exceptions. Some were using Fog::Rackspace::Errors::NotFound other were using Fog::Rackspace::Service::NotFound and still others were using Fog::Service::Rackspace::NotFound.

Based on my conversation with @geemus, I decided to redo all NotFound exceptions to use Fog::Service::Rackspace::NotFound. I couldn't find anyway to make this backwards compatible with the NotFound exception being created when each service inherits from Fog::Service.

Let me know if this is going to pose a big issue and I will re-implement this.

Exception discussion:
https://groups.google.com/forum/?fromgroups=#!topic/ruby-fog/NKbZwEijf9w

NotFound class is created when Fog::Core is inherited:
https://github.com/fog/fog/blob/master/lib/fog/core/service.rb#L39