add a task to check for warnings and fail if there were any

We use grunt continue to run some tests and clean up the db no matter what. However we want the overall run to fail so that we are notified. 

To fix this I've added an optional task: `continueFailIfWarningsWereIssued` that you can add to the end of a run, if any warnings were issued the task will be aborted and you'll get `6` as your exit code.