Properly handle DOI limit reached

[mfenner]

ALM Reports raises an error for queries with more than 2,000 DOIs. This should be handled gracefully.

[FATAL] DoiLimitReachedError (Reached limit of 2000 DOIs):
  app/controllers/application_controller.rb:31:in `[]='
  app/controllers/id_controller.rb:222:in `block in process_upload'
  app/controllers/id_controller.rb:217:in `each'
  app/controllers/id_controller.rb:217:in `process_upload'
  app/controllers/application_controller.rb:105:in `save_session_dois' (pid:27511)

[mfenner]

This error is related, and described in #37.

NoMethodError (undefined method `length' for nil:NilClass):
  app/controllers/application_controller.rb:130:in `articleLimitReached?'
  app/controllers/id_controller.rb:14:in `index'
  app/controllers/application_controller.rb:105:in `save_session_dois' (pid:27390)