Fix NoMethodError: undefined method `backtrace' for nil:NilClass

There are errors without exception object. The backtrace is

```
NoMethodError: undefined method `backtrace' for nil:NilClass
... 10 non-project frames
1
File "[/project/]/shared/bundle/ruby/2.1.0/gems/sidekiq-3.2.1/lib/sidekiq/exception_handler.rb" line 10 in call
2
File "[/project/]/shared/bundle/ruby/2.1.0/gems/sidekiq-3.2.1/lib/sidekiq/exception_handler.rb" line 20 in block in handle_exception
3
File "[/project/]/shared/bundle/ruby/2.1.0/gems/sidekiq-3.2.1/lib/sidekiq/exception_handler.rb" line 18 in each
4
File "[/project/]/shared/bundle/ruby/2.1.0/gems/sidekiq-3.2.1/lib/sidekiq/exception_handler.rb" line 18 in handle_exception
5
File "[/project/]shared/bundle/ruby/2.1.0/gems/sidekiq-3.2.1/lib/sidekiq/launcher.rb" line 31 in actor_died
6
File "[/project/]/shared/bundle/ruby/2.1.0/gems/celluloid-0.15.2/lib/celluloid/actor.rb" line 357 in handle_exit_event
7
File "[/project/]/shared/bundle/ruby/2.1.0/gems/celluloid-0.15.2/lib/celluloid/actor.rb" line 340 in block in handle_system_event
8
File "[/project/]/shared/bundle/ruby/2.1.0/gems/celluloid-0.15.2/lib/celluloid/actor.rb" line 416 in block in task
9
File "[/project/]/shared/bundle/ruby/2.1.0/gems/celluloid-0.15.2/lib/celluloid/tasks.rb" line 55 in block in initialize
10
File "[/project/]/shared/bundle/ruby/2.1.0/gems/celluloid-0.15.2/lib/celluloid/tasks/task_fiber.rb" line 13 in block in create
```