Discard jobs where the record cannot be found

[bbbriana]

https://github.com/shop/issues/issues/1596

If there are no records, we cannot take any action so discard the job:

The job cannot function without the record:                                                                                                                                                                      
                                                                                                                                                                                                                   
  The RecordNotFound is raised during GlobalID deserialization, before perform is even called. But even if it weren't, look at what execute does immediately:                                                      
                                                                                                                                                                                                                   
  def execute                                                                                                                                                                                                      
    @commands = Commands.for(@task)      # needs task                                                                                                                                                              
    unless @task.pending?                 # needs task                                                                                                                                                             
      Rails.logger.error("Task ##{@task.id}...")  # needs task                                                                                                                                                     
      return                                                                                                                                                                                                       
    end                                                                                                                                                                                                            
    run                                                                                                                                                                                                            
                                                                                                                                                                                                                   
  And run calls:                                                                                                                                                                                                   
  - @task.ping                                                                                                                                                                                                     
  - @task.run!                                                                                                                                                                                                     
  - @task.write(...)                                                                                                                                                                                               
  - @task.report_complete!                                                                                                                                                                                         
  - etc.                                                                                                                                                                                                           
                                                                                                                                                                                                                   
  Every single operation requires the task record. Without it:                                                                                                                                                     
  - No commands can be built                                                                                                                                                                                       
  - No status can be checked                                                                                                                                                                                       
  - No output can be written                                                                                                                                                                                       
  - No completion can be reported        

[bbbriana]

Closed in favor of a PR with a smaller blast radius: https://github.com/Shopify/shipit/pull/3433