Fix image error msg not appearing.

ECOM-7858

course_discovery/static/js/publisher/image-validation.js

 $(function () {
     $('#id_image').on('change', function () {
         // max size limit is 1 MB
-        var maxFileSize = 1000000;
+        var maxFileSize = 1000000,
+            $imageError = $('.image-error');
 
         if (this.files && this.files[0]) {
             if (this.files[0].size > maxFileSize) {
-               $('.image-error').append(gettext('The image file size cannot exceed 1 MB.'));
+               $imageError.html(gettext('The image file size cannot exceed 1 MB.'));
                 $('#id_image').val("");
+                $imageError.show();
+                $('.course-image-field .errorlist').closest('.has-error').hide();
             } else {
-                $('.image-error').empty();
+                $imageError.hide();
             }
         }
     });

course_discovery/templates/publisher/course_edit_form.html

@@ -340,6 +340,7 @@
                                         <label class="field-label ">{{ form.image.label_tag }}</label>
                                         <div class="course-image-field">
                                             {{ form.image }}
+                                            <div class="image-error has-error hidden"></div>
                                             {% if form.image.errors %}
                                                 <div class="field-message has-error">
                                                     <span class="field-message-content">