FEATURE: Add site setting to show more detailed 404 errors. (#8014)

If the setting is turned on, then the user will receive information
about the subject: if it was deleted or requires some special access to
a group (only if the group is public). Otherwise, the user will receive
a generic #404 error message. For now, this change affects only the
topics and categories controller.

This commit also tries to refactor some of the code related to error
handling. To make error pages more consistent (design-wise), the actual
error page will be rendered server-side.

lib/discourse.rb

@@ -71,13 +71,18 @@ module Discourse
 
   # When they don't have permission to do something
   class InvalidAccess < StandardError
-    attr_reader :obj, :custom_message, :opts
+    attr_reader :obj
+    attr_reader :opts
+    attr_reader :custom_message
+    attr_reader :group
+
     def initialize(msg = nil, obj = nil, opts = nil)
       super(msg)
 
       @opts = opts || {}
-      @custom_message = opts[:custom_message] if @opts[:custom_message]
       @obj = obj
+      @custom_message = opts[:custom_message] if @opts[:custom_message]
+      @group = opts[:group] if @opts[:group]
     end
   end
 
@@ -86,12 +91,15 @@ module Discourse
     attr_reader :status
     attr_reader :check_permalinks
     attr_reader :original_path
+    attr_reader :custom_message
+
+    def initialize(msg = nil, status: 404, check_permalinks: false, original_path: nil, custom_message: nil)
+      super(msg)
 
-    def initialize(message = nil, status: 404, check_permalinks: false, original_path: nil)
       @status = status
       @check_permalinks = check_permalinks
       @original_path = original_path
-      super(message)
+      @custom_message = custom_message
     end
   end