Raised when a key is expected to be present in the request parameters, but is not.

This exception is raised by Phoenix.Controller.scrub_params/2 which:

• Checks to see if the required_key is present (can be empty)
• Changes all empty parameters to nils (“” -> nil).

If you are seeing this error, you should handle the error and surface it to the end user. It means that there is a parameter missing from the request.