FIX: show rejected emails with unrecognized errors (#5026)

Although 407a23663df6c8e66c5c3b8e5cbf21a609de4109 will send rejection
messages for unrecognized errors, sometimes processing the email will
raise an error which has a blank message.

This commit:

1. Shows rejected emails which have already been processed and contain
   a blank error in /admin/email/rejected

2. Replaces new blank error messages with the error type

lib/email/receiver.rb

@@ -49,7 +49,9 @@ module Email
           @incoming_email = create_incoming_email
           process_internal
         rescue => e
-          @incoming_email.update_columns(error: e.to_s) if @incoming_email
+          error = e.to_s
+          error = e.class.name if error.blank?
+          @incoming_email.update_columns(error: error) if @incoming_email
           raise
         end
       end