Extract formValidate class name from JavaScript

It is now provided via the data-form-validate-error-class attribute.

h/static/scripts/directives.coffee

 formValidate = ->
   link: (scope, elem, attr, form) ->
+    errorClassName = attr.formValidateErrorClass
+
     toggleClass = (field, {addClass}) ->
       fieldEl = elem.find("[data-target=#{field.$name}]")
-      fieldEl.toggleClass('form-field-error', addClass)
+      fieldEl.toggleClass(errorClassName, addClass)
 
     updateField = (field) ->
       return unless field?

tests/js/directives-test.coffee

@@ -34,7 +34,7 @@ describe 'h.directives', ->
       $scope.model = {username: ''}
 
       template = '''
-      <form form-validate name="login" onsubmit="return false">
+      <form form-validate data-form-validate-error-class="form-field-error" name="login" onsubmit="return false">
         <div class="form-field" data-error-class="form-field-error" data-target="username">
           <input type="text" class="" ng-model="model.username" name="username" required ng-minlength="3" />
         </div>