Add error handling to form labels

This re-enables an `-error` state for labels when the associated
field is erroneous.

Also shows an error icon next to the label and changes the font style.

app/assets/stylesheets/content/_forms.sass

@@ -342,6 +342,17 @@ fieldset.form--fieldset
     @include grid-size(shrink)
     max-width: none
 
+  &.-error
+    @extend .icon-error
+    color: $content-form-danger-zone-bg-color
+    font-weight: bold
+    &::before
+      @include icon-common
+      display: inline-block
+      line-height:  $base-line-height
+      padding-right: 0.325rem
+
+
   &.-required,
   .form--field.-required > &
     &::after

lib/tabular_form_builder.rb

@@ -198,9 +198,10 @@ class TabularFormBuilder < ActionView::Helpers::FormBuilder
 
     id = element_id(translation_form) if translation_form
 
-    # FIXME: reenable the error handling
-    label_options[:class] << 'error' if false && @object && @object.respond_to?(:errors) && @object.errors[field] # FIXME
     label_options[:class] << 'form--label'
+
+    has_errors = @object.try(:errors) && @object.errors.include?(field)
+    label_options[:class] << ' -error' if has_errors
     label_options[:class] << ' -required' if options.delete(:required)
     label_options[:for] = if options[:for]
                             options[:for]

spec/lib/tabular_form_builder_spec.rb

@@ -582,7 +582,7 @@ JJ Abrams</textarea>
         end
       end
 
-      context 'with ActiveModel and withouth specified label' do
+      context 'with ActiveModel and without specified label' do
         let(:resource) {
           FactoryGirl.build_stubbed(:user,
                                     firstname:  'JJ',
@@ -595,6 +595,18 @@ JJ Abrams</textarea>
         it 'uses the human attibute name' do
           expected_label_like(User.human_attribute_name(:name))
         end
+
+        context 'with erroneous field' do
+          before do
+            resource.errors.add(:name, :invalid)
+          end
+
+          it 'shows an appropriate error label' do
+            expect(output).to have_selector 'label.-error',
+                                            count: 1,
+                                            text: User.human_attribute_name(:name)
+          end
+        end
       end
 
       context 'when required, with a label specified as symbol' do