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

app/models/incoming_email.rb

@@ -3,7 +3,7 @@ class IncomingEmail < ActiveRecord::Base
   belongs_to :topic
   belongs_to :post
 
-  scope :errored,  -> { where("NOT is_bounce AND LENGTH(COALESCE(error,'')) > 0") }
+  scope :errored,  -> { where("NOT is_bounce AND error IS NOT NULL") }
 end
 
 # == Schema Information