GNOME Software reports unknown error when there is a signing issue
If you have packages in your update queue that are unsigned (or have an invalid signature), PackageKit reports "Unable to download updates: could not do untrusted question as no klass support".
Digging into packagekitd in --verbose mode, it shows emitting error-code bad-gpg-signature, 'failed to lookup digest in keyring for <redacted>
The proper error should be passed up and the offending package should be indicated.
Ideally, GNOME Software should still at least offer to update any packages that have no signing issues.
Edited by Philip Withnall