do not generate exception/backtrace

[kares]

... such as :

2017-05-05T07:53:29.560+02:00 [platform (#f719ad0)-jruby-rack-worker#1] INFO TraceType : Exception raised: NoMethodError : undefined method `to_f' for false:FalseClass
2017-05-05T07:53:29.561+02:00 [platform (#f719ad0)-jruby-rack-worker#1] INFO TraceType : Backtrace generated:
      calc_sleep_time at uri:classloader:/delayed/sleep_calculator.rb:21
                sleep at uri:classloader:/delayed/sleep_calculator.rb:6
       block in start at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/delayed_job-4.1.2/lib/delayed/worker.rb:186
                 loop at org/jruby/RubyKernel.java:1298
       block in start at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/delayed_job-4.1.2/lib/delayed/worker.rb:172
  block in ClearLocks at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/delayed_job-4.1.2/lib/delayed/plugins/clear_locks.rb:7
         block in add at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/delayed_job-4.1.2/lib/delayed/lifecycle.rb:79
  block in initialize at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/delayed_job-4.1.2/lib/delayed/lifecycle.rb:61
         block in add at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/delayed_job-4.1.2/lib/delayed/lifecycle.rb:79
              execute at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/delayed_job-4.1.2/lib/delayed/lifecycle.rb:66
        run_callbacks at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/delayed_job-4.1.2/lib/delayed/lifecycle.rb:40
                start at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/delayed_job-4.1.2/lib/delayed/worker.rb:171
               <main> at uri:classloader:/delayed/start_worker.rb:19
                 load at org/jruby/RubyKernel.java:979
      block in (root) at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:1
      load_dependency at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:240
                 load at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:268
               <main> at /opt/tomcat/webapps/smb/WEB-INF/gems/gems/activesupport-4.2.6/lib/active_support/dependencies.rb:268

0.13.2 + DJ 4.1.2