Added an error message in the DebugClassLoader when using / instead of \.

Using ``/`` instead of ``\`` when writing class names in a configuration seems to happen regularly. See https://github.com/symfony/symfony/commit/aba8f1e1802fbafcd954c2b6f59272492bd4e6f7#commitcomment-2461266 for the latest case I saw. I guess it may come from the fact that SensioGeneratorBundle allows using both for convenience in the CLI to get rid of escaping issues.
This will give people a better error message when they use a ``/`` instead of having them figure that the class name in the message using ``/`` and not ``\`` and this is the reason of the issue.