[How to] fix NO_PUBKEY error

Sometimes a manually added PPA could cause update error which looks like below during the update:

The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXXXX

This can be fixed by issuing the following 2 commands in terminal:

gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv XXXXXXXXXX

gpg --export --armor XXXXXXXXXX | sudo apt-key add -