W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://download.owncloud.org/download/repositories/9.0/Ubuntu_16.04  Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 47AE7F72479BC94B
W: Failed to fetch http://download.owncloud.org/download/repositories/9.0/Ubuntu_16.04/Release.gpg  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 47AE7F72479BC94B
W: Some index files failed to download. They have been ignored, or old ones used instead.