Implement task status "error"

If a task crashes on import, the task status should be set to "crashed". In addition it should be deleteable in the task overview. It's color in the overview should be red. The color of a not-assigned task should switch from red to purple.

All exceptions during import must lead to this status, except those in termTagger-Workers. This could be achieved by using the php-method
http://php.net/manual/en/function.register-shutdown-function.php
in the ErrorController. This method could look up the task-status and if it is "importing" switch it to "crashed" - except if the error is triggered by the termTagger-Plugin.

If a task cannot be imported because of errors, it should get the state crashed. Implement for simple / obvious cases first.