Fix recursion when Notice#email_notification send fails

When a notification can't be sent, errbit tries to send a notification that the notification failed, which results in a nasty recursion that won't stop soon.

In my case it was a simple `Net::SMTPFatalError: 554 5.7.1 <user@example.com>: Relay access denied` which caused lots of load.

A sample stacktrace which extends repeats in the middle part

```
/usr/lib/ruby/2.0.0/net/smtp.rb:950→ check_response
/usr/lib/ruby/2.0.0/net/smtp.rb:919→ getok
/usr/lib/ruby/2.0.0/net/smtp.rb:862→ rcptto
/usr/lib/ruby/2.0.0/net/smtp.rb:843→ block in rcptto_list
...
activesupport-3.2.15/lib/active_support/notifications/instrumenter.rb:20→ instrument
activesupport-3.2.15/lib/active_support/notifications.rb:123→ instrument
actionmailer-3.2.15/lib/action_mailer/base.rb:413→ deliver_mail
mail-2.5.4/lib/mail/message.rb:232→ deliver
app/models/notice.rb:166→ email_notification
activesupport-3.2.15/lib/active_support/callbacks.rb:435→ _run__FRAGMENT__create__FRAGMENT__callbacks
activesupport-3.2.15/lib/active_support/callbacks.rb:405→ __run_callback
activesupport-3.2.15/lib/active_support/callbacks.rb:385→ _run_create_callbacks
activesupport-3.2.15/lib/active_support/callbacks.rb:81→ run_callbacks
...
mongoid-3.1.5/lib/mongoid/persistence/operations/insert.rb:26→ persist
mongoid-3.1.5/lib/mongoid/persistence.rb:51→ insert
mongoid-3.1.5/lib/mongoid/persistence.rb:80→ save
mongoid-3.1.5/lib/mongoid/relations/referenced/many.rb:35→ <<
app/models/error_report.rb:52→ generate_notice!
lib/overrides/hoptoad_notifier/hoptoad_notifier.rb:18→ send_notice
app/models/notice.rb:168→ rescue in email_notification
app/models/notice.rb:165→ email_notification
activesupport-3.2.15/lib/active_support/callbacks.rb:435→ _run__FRAGMENT__create__FRAGMENT__callbacks
activesupport-3.2.15/lib/active_support/callbacks.rb:405→ __run_callback
activesupport-3.2.15/lib/active_support/callbacks.rb:385→ _run_create_callbacks
activesupport-3.2.15/lib/active_support/callbacks.rb:81→ run_callbacks
...
mongoid-3.1.5/lib/mongoid/persistence/operations/insert.rb:26→ persist
mongoid-3.1.5/lib/mongoid/persistence.rb:51→ insert
mongoid-3.1.5/lib/mongoid/persistence.rb:80→ save
mongoid-3.1.5/lib/mongoid/relations/referenced/many.rb:35→ <<
app/models/error_report.rb:52→ generate_notice!
app/controllers/notices_controller.rb:14→ create
actionpack-3.2.15/lib/action_controller/metal/implicit_render.rb:4→ send_action
actionpack-3.2.15/lib/abstract_controller/base.rb:167→ process_action
actionpack-3.2.15/lib/action_controller/metal/rendering.rb:10→ process_action
actionpack-3.2.15/lib/abstract_controller/callbacks.rb:18→ block in process_action
...
/var/lib/gems/1.9.1/gems/passenger-3.0.19/lib/phusion_passenger/spawn_manager.rb:275→ handle_spawn_application
/var/lib/gems/1.9.1/gems/passenger-3.0.19/lib/phusion_passenger/abstract_server.rb:357→ server_main_loop
/var/lib/gems/1.9.1/gems/passenger-3.0.19/lib/phusion_passenger/abstract_server.rb:206→ start_synchronously
/var/lib/gems/1.9.1/gems/passenger-3.0.19//helper-scripts/passenger-spawn-server:99→ <main>
```