* lib/razor/task.rb (all): fix a few problems

  * what should have been a list of names was a list of names for file-baed
    installers and a list of DB objects for DB based installers
  * if a task was invalid, the whole method would just throw an error;
    since the server will otherwise function happily even if a task in the
    filesystem is invalid, we will just not include such tasks in the
    returned list

Fixes https://tickets.puppetlabs.com/browse/RAZOR-109