Frontend: Proper error display when adding a new user

[laredotornado]

When we add a new user, we need to have errors display when the form has not been filled out correctly. The style of the errors shoudl mirror the pattern we used when submitting a new coop (https://map.chicommons.coop/directory-additions-updates). An example of the JSON returned when a new user form is submitted incorrectly would be

{"email":["This field is required."]}