Treat integrity check failure as a warning

[kettanaito]

Originating from #81, I think that treating integrity check failure as an error and disabling the mocking is a safe, but too extreme measure for the users.

Expected behavior

• Integrity check failure is still treated as an error, for the purpose of giving it a higher priority when the user browses's his console output.
• Integrity check failure does not disable the mocking, for the purpose of changes that do not necessarily break backwards-compatibility. It's up to user to update the Service Worker, although it's highly recommended.