First I'd like to re-iterate @userabuser. You should never edit the core files of WordPress. In fact, you should avoid editing plug-ins and themes (use a child-theme instead) since changes are lost on an update, and you should update!
I would follow the method outlined in these questions:
- http://wordpress.stackexchange.com/questions/48785/custom-taxonomy-as-categories-remove-most-used-tab/48816#48816Custom-Taxonomy as categories: Remove "most-used" tab?
- http://wordpress.stackexchange.com/questions/48937/custom-post-type-taxonomies-ui-radiobuttons-not-checkboxes/48942#48942custom post type taxonomies UI radiobuttons not checkboxes
and de-register the existing metabox used (I think as @userabuser uses, the ID is
#tagsdiv-post_tag). Then re-register it, allowing you to create a function which defines its insides as explained in the above posts.
For your purposes however, the function you want to mimic is
post_tags_meta_box(), found here: http://core.trac.wordpress.org/browser/tags/3.3/wp-admin/includes/meta-boxes.php#L265
Simply use the contents of that function in your own custom call back, and edit to display your messages.