Change reject_bulk and destroy methods

Create a private method to define the new instance of UserDestroyer. The
reject_bulk method now uses the group_by method to collect all the
results of the check into a Hash. This way, it just collects all the
true values to get the successes and subtracts that number to the total
number of users for the failures.

The destroy method now uses a top-level rescue clause to make the method
a little more readable.