Add PermissionDeniedError error class under GDS::SSO namespace

- Adds a warning log message that the existing PermissionDeniedException should be depracated on a new major release, but it will still work for current apps.
  The reason is that the new error GDS::SSO::PermissionDeniedError has been created to replace the existing GDS::SSO::ControllerMethods::PermissionDeniedException. Also the new error has been placed on the outer layer of the name-spacing (GDS::SSO), which is more generic than the existing GDS::SSO::ControllerMethods one. And it follows the Rails conventions that the error class names should end with ...Error, which is more indicative that they inherit from StandardError rather than the top level error class Exception.
- Updates the Railtie so any rescue errors from PermissionDeniedError will be transformed to :forbidden Rails specific ones.

lib/gds-sso.rb

@@ -15,6 +15,7 @@ module SSO
     autoload :User,                     "gds-sso/user"
     autoload :ApiAccess,                "gds-sso/api_access"
     autoload :AuthorisedUserConstraint, "gds-sso/authorised_user_constraint"
+    autoload :PermissionDeniedError,    "gds-sso/controller_methods"
 
     # User to return as logged in during tests
     mattr_accessor :test_user

lib/gds-sso/controller_methods.rb

@@ -1,11 +1,19 @@
 module GDS
   module SSO
+    class PermissionDeniedError < StandardError
+    end
+
     module ControllerMethods
-      class PermissionDeniedException < StandardError
+      # TODO: remove this for the next major release
+      class PermissionDeniedException < PermissionDeniedError
+        def initialize(...)
+          warn "GDS::SSO::ControllerMethods::PermissionDeniedException is deprecated, please replace with GDS::SSO::PermissionDeniedError"
+          super(...)
+        end
       end
 
       def self.included(base)
-        base.rescue_from PermissionDeniedException do |e|
+        base.rescue_from PermissionDeniedError do |e|
           if GDS::SSO::Config.api_only
             render json: { message: e.message }, status: :forbidden
           else

lib/gds-sso/railtie.rb

@@ -7,6 +7,10 @@ class Railtie < Rails::Railtie
           config.api_only = Rails.configuration.api_only
         end
         OmniAuth.config.logger = Rails.logger
+
+        config.action_dispatch.rescue_responses.merge!(
+          "GDS::SSO::PermissionDeniedError" => :forbidden,
+        )
       end
     end
   end