Suppose you have this route:

homepage:
    pattern:  /
    defaults: { _controller: "FooBundle:Course2:index" }

But, no Course2Controller exists (you didn't create it yet, typo in its namespace, etc). The current error looks like this:

The FooBundle:Course2 is a bit confusing, because I don't recognize that from anywhere (I typed FooBundle:Course2:index). I'd like to improve this error, perhaps:

The _controller value "FooBundle:Course2:index" maps to a "Knp\FooBundle\Controller\Course2Controller" class, but this class was not found. Create this class or check the spelling of the class and its namespace.