be more clear about blocks not working with accessible_by - closes #130

lib/cancan/ability.rb

@@ -216,7 +216,7 @@ module CanCan
     def relevant_can_definitions_for_query(action, subject)
       relevant_can_definitions(action, subject).each do |can_definition|
         if can_definition.only_block?
-          raise Error, "Cannot determine SQL conditions or joins from block for #{action.inspect} #{subject.inspect}"
+          raise Error, "The accessible_by call cannot be used with a block 'can' definition. The SQL cannot be determined for #{action.inspect} #{subject.inspect}"
         end
       end
     end