Bug 1030972 - Import tasks from Bugzilla

@bobsilverberg @Osmose r?
Simply logging (which is still a `TODO`) isn't really going to help when a user encounters an error. They won't know why their query seemed to return no records. I think it would be better to throw an error which can either bubble up to the user, or can be caught by you at a higher level and presented in a friendlier way.