fix deliver_via_email for ruby 1.8.

fixes CNVS-4196

Timeout::Error doesn't inherit from StandardError in
< 1.9, causing some exceptions to not be properly handled
in deliver_via_email.

test plan:
  * run spec/lib/message_dispatcher_spec.rb with ruby 1.8
    and verify that specs pass.

Change-Id: I233c62a5960e27804ef37f529512bb39b8746239
Reviewed-on: https://gerrit.instructure.com/18079
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Bryan Madsen <bryan@instructure.com>
Reviewed-by: Eric Berry <ericb@instructure.com>
Tested-by: Eric Berry <ericb@instructure.com>

app/models/message.rb

@@ -609,7 +609,7 @@ class Message < ActiveRecord::Base
       @exception = e
       logger.error "Exception: #{e.class}: #{e.message}\n\t#{e.backtrace.join("\n\t")}"
       cancel if e.message.try(:match, /Bad recipient/)
-    rescue => e
+    rescue StandardError, Timeout::Error => e
       @exception = e
       logger.error "Exception: #{e.class}: #{e.message}\n\t#{e.backtrace.join("\n\t")}"
     end