HTC update.zip is encrypted and does not respect the standard file layout. META-INF is missing, content is:
Therefore this file can only flashed from stock recovery.
I want to preface this by saying that the following is what worked for me. I can't guarantee it will work for you, but I wanted to document it here for the future in case I need it again or someone else finds it useful. I take no responsibility the possibility of this messing up your device even worse or not doing anything at all. I don't really understand ...
You are on the right track.
The message "failed to verify whole-file signature", means that the file could not be deemed valid for the provided certificate.
The missing error message indicates that this is the case as the signature could not be matched to the trusted certificate store: