Example of error: GPG error: http://ppa.launchpad.net trusty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4DF9B28CA252A784
Fix with this:
$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <KEY>
Refer: http://askubuntu.com/questions/520828/gpg-error-no-pubkey-warning-the-following-packages-cannot-be-authenticated