Make errors more inspectable by settings message

When message is set as variable, instead of redefining `message`
method, the message will be used in `inspect` and `to_s` calls,
both implicit and explicit, which improves logs readability and
simmplifies debugging in the irb

lib/aasm/errors.rb

@@ -7,10 +7,7 @@ module AASM
 
     def initialize(object, event_name, state_machine_name, failures = [])
       @object, @event_name, @originating_state, @failures = object, event_name, object.aasm(state_machine_name).current_state, failures
-    end
-
-    def message
-      "Event '#{event_name}' cannot transition from '#{originating_state}'. #{reasoning}"
+      super("Event '#{event_name}' cannot transition from '#{originating_state}'. #{reasoning}")
     end
 
     def reasoning