Added fallback message if ffmpeg is not found

wagtailvideos/templates/wagtailvideos/videos/edit.html

@@ -54,6 +54,9 @@
                     </li>
                 </ul>
             </form>
+        {% else %}
+            <br/><br/>
+            <span class='transcode-error'>Ffmpeg is not found on your server. Please install if you wish to transcode videos into an HTML5 video compliant format.</span>
         {% endif %}
     </div>
     <div class="col2 ">