This worked, thanks.
It’s not an error, it’s purely informational. Your GPG instance doesn’t have one of the keys used to sign a key, which can (for example) be caused by a third-party (non-dev) signing the key.
Gpg: invalid radix64 character * skipped
Signatures not checked due to missing keys
You’re right though, this needs to be made obvious to users - it made me question when it first appeared, and there are several posts on the Arch forums when it happened with their keyrings.