Catch 'invalid reoccurence' error and reset schedule type

![image](https://f.cloud.github.com/assets/494927/847090/f290e4ec-f431-11e2-89af-5072e95f0673.png)

* BackUpWordPress has been active in the past.
* BackUpWordPress currently inactive but WP Remote active.
* I updated BackUpWordPress to the latest version
* I activated BackUpWordPress & went to the page in the admin
* Default schedules have "Invalid Reoccurence" and cannot be deleted
* I can still create new schedules fine