title: How to validate input and raise fault in Apigee
body: <p>How can I validate an input to APIGee endpoint and raise a fault if input is not provided or if it is not in a valid format?</p>
  
  <p>For eg: I am expecting a value to be passed in querystring. If its not supplied, I should return a error response instead of letting it pass through back-end service and fail there.</p>
  
  <p>Thanks,
  Deepak</p>