Problèmes d'installation de unetbootin sur ubuntu 16.04

J'ai couru

sudo add-apt-repository ppa:gezakovacs/ppa

et j'obtiens le résultat suivant avec les 4 dernières lignes donnant des erreurs:-

gpg: keyring `/tmp/tmp8k7uho4o/secring.gpg' created
gpg: keyring `/tmp/tmp8k7uho4o/pubring.gpg' created
gpg: requesting key FC91AE7E from hkp server keyserver.ubuntu.com
gpg: /tmp/tmp8k7uho4o/trustdb.gpg: trustdb created
gpg: key FC91AE7E: public key "Launchpad PPA for Geza Kovacs" imported
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)
gpg: keyblock resource `/etc/apt/trusted.gpg.d/gezakovacs_ubuntu_ppa.gpg': file open error
gpg: no writable keyring found: eof
gpg: error reading `/tmp/tmp8k7uho4o/pubring.gpg': general error
gpg: import from `/tmp/tmp8k7uho4o/pubring.gpg' failed: general error

Merci beaucoup pour toute aide

Ajout de sorties d'écran selon les besoins

$ sudo apt update
[sudo] password for kerry: 
Get:1 http://ppa.launchpad.net/gezakovacs/ppa/ubuntu xenial InRelease [17.5 kB]
Get:2 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
Err:1 http://ppa.launchpad.net/gezakovacs/ppa/ubuntu xenial InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D45DF2E8FC91AE7E
Err:2 http://security.ubuntu.com/ubuntu xenial-security InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5  NO_PUBKEY 3B4FE6ACC0B21F32
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net/gezakovacs/ppa/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY D45DF2E8FC91AE7E
E: The repository 'http://ppa.launchpad.net/gezakovacs/ppa/ubuntu xenial InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://security.ubuntu.com/ubuntu xenial-security InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5  NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://security.ubuntu.com/ubuntu xenial-security InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

Pour David Foerster :-

$ apt-key list
/etc/apt/trusted.gpg
--------------------
pub   1024D/7FAC5991 2007-03-08
uid                  Google, Inc. Linux Package Signing Key <linux-packages-keymaster@google.com>
sub   2048g/C07CB649 2007-03-08

pub   4096R/D38B4796 2016-04-12
uid                  Google Inc. (Linux Packages Signing Authority) <linux-packages-keymaster@google.com>
sub   4096R/640DB551 2016-04-12 [expires: 2019-04-12]

と

ls -l /etc/apt/trusted.gpg*
-rw-r--r-- 1 root root 4171 Jan 15 07:13 /etc/apt/trusted.gpg
-rw-r--r-- 1 root root 1203 Jan 15 07:13 /etc/apt/trusted.gpg~

と

awk '$2=="/"' /proc/mounts
/dev/mapper/ubuntu--vg-root / ext4 rw,relatime,errors=remount-ro,data=ordered 0 0