UndefinedDecisionError is a StandardError

To raise as a exception, the class requires to be a Exception.
Usual Exception should be a StandardError, which is captured by
rescue-clause by default.

lib/perfectqueue/application/decider.rb

@@ -19,7 +19,7 @@
 module PerfectQueue
   module Application
 
-    class UndefinedDecisionError
+    class UndefinedDecisionError < StandardError
     end
 
     class Decider