core: improve error message in case of wrong return types

at the moment the follwoing code produces an error which is not
really addressing the issue

Robot.actions do
    describe('move robot')
    def move(arg)
        Roby::Task.new
    end
end
--> invalid replacement: missing provided models
Roby2::Actions::Main::Move

lib/roby/actions/models/action.rb

@@ -199,6 +199,9 @@ def run(action_interface, arguments = Hash.new)
                         raise ArgumentError, "#{name} expects no arguments, but #{arguments.size} are given"
                     end
                     result = action_interface.send(name).as_plan
+                    if(!result.class.has_ancestor?(returned_type))
+                        raise ArgumentError, "#{name} is expected to return #{returned_type} but returned #{result.class}"
+                    end
                 else
                     default_arguments = self.arguments.inject(Hash.new) do |h, arg|
                         h[arg.name] = arg.default
@@ -211,6 +214,9 @@ def run(action_interface, arguments = Hash.new)
                         end
                     end
                     result = action_interface.send(name, arguments).as_plan
+                    if(!result.class.has_ancestor?(returned_type))
+                        raise ArgumentError, "#{name} is expected to return #{returned_type} but returned #{result.class}"
+                    end
                 end
                 # Make the planning task inherit the model/argument flags
                 if planning_task = result.planning_task

lib/roby/relations/dependency.rb

@@ -921,7 +921,7 @@ def pretty_print(pp) # :nodoc:
             if mode == :failed_event
                 pp.text "triggered the failure predicate '#{relation[:failure]}': "
             elsif mode == :unreachable_success
-                pp.text "cannot reach the success condition '#{relation[:success]}': "
+                pp.text "success condition can no longer be reached '#{relation[:success]}': "
             end
             explanation.pretty_print(pp)