Modify(userform.tpl.html): abandon generating form fields with ng-repeat - define fields staticly.
Modify(userForm.js): remove factory 'formFields' - use formData and formErrors for storing input data and for error messages.
Modify(userForm.js): add function validateField to factory formErrors; add flags submitted and usernameExists to factory formData.
Modify(userform.tpl.html): use the validateField function to shorten the validation checks.