Display Document*ingEvent cancel reason in the UI

After XWIKI-9054 and XWIKI-15391, Document*ingEvent(s) are now cancelable by a listener. However, even if the cancel reason is provided, the UI does not relay this and the user usually gets a generic failure notification. The reason only gets logged in the server logs, outside the reach of the UI.

Example usecase where the user should be notified of the cancel reason:

• when uploading an attachment, the antivirus application detects an infection and cancels the upload,
• when creating a user, the limits application cancels the user creation because the users limit was exceeded
• when uploading an attachment, if there is not enough storage space,
• etc.