Displaying price validation errors

Added the necessary DOM elements and an internationalized error message.

ECOM-2075

ecommerce/static/js/models/course_seats/course_seat.js

@@ -15,7 +15,8 @@ define([
 
             validation: {
                 price: {
-                    required: true
+                    required: true,
+                    msg: gettext('All course seats must have a price.')
                 },
                 product_class: {
                     oneOf: ['Seat']

ecommerce/static/templates/professional_course_seat_form_field.html

@@ -13,6 +13,8 @@
                 <input type="number" class="form-control" name="price" id="price" min="5" step="1" pattern="\d+"
                        value="<%= price %>">
             </div>
+            <!-- NOTE: This help-block is here for validation messages. -->
+            <span class="help-block"></span>
         </div>
     </div>
 

ecommerce/static/templates/verified_course_seat_form_field.html

@@ -14,6 +14,8 @@
                 <input type="number" class="form-control" name="price" id="price" min="5" step="1" pattern="\d+"
                        value="<%= price %>">
             </div>
+            <!-- NOTE: This help-block is here for validation messages. -->
+            <span class="help-block"></span>
         </div>
     </div>