Fix message in nuxeo-tag-element element when tag is already added

When trying to add a tag that is already added to the document, the message in the drop down is misleading.

How to reproduce:

• Open Asset
• Create a tag or a few tags in the Tags section
• Create a tag with the same name as the existing tag added to Asset
• Observe message on tags dropdown

Expected result: the message has to inform that the user cannot create a tag with the same name.
Actual Result: the message shows No result for <tag name>, It is misleading because a tag with this name already exists. See screenshot.