* move exception message to exception constructor

* save original exception
* keep original backtrace

actionpack/lib/action_dispatch/middleware/session/abstract_store.rb

@@ -7,6 +7,15 @@
 module ActionDispatch
   module Session
     class SessionRestoreError < StandardError #:nodoc:
+      attr_reader :original_exception
+
+      def initialize(const_error)
+        @original_exception = const_error
+
+        super("Session contains objects whose class definition isn't available.\n" +
+          "Remember to require the classes for all objects kept in the session.\n" +
+          "(Original exception: #{const_error.message} [#{const_error.class}])\n")
+      end
     end
 
     module DestroyableSession
@@ -58,11 +67,8 @@ def stale_session_check!
           begin
             # Note that the regexp does not allow $1 to end with a ':'
             $1.constantize
-          rescue LoadError, NameError => const_error
-            raise ActionDispatch::Session::SessionRestoreError,
-              "Session contains objects whose class definition isn't available.\n" +
-              "Remember to require the classes for all objects kept in the session.\n" +
-              "(Original exception: #{const_error.message} [#{const_error.class}])\n"
+          rescue LoadError, NameError => e
+            raise ActionDispatch::Session::SessionRestoreError, e, e.backtrace
           end
           retry
         else