> `%s` to print `err` or `(cdr err)` would be wrong, since `%s` is for use
> with strings rather than lists.  IOW, IMO, it should be either
>
>     ...%S" ... err)
>
> or
>
>     ...%s" ... (error-message-string err))
>
> where the first is a bit more "debugging/developer" friendly and the second
> is a bit more "user" friendly.

Revising the code of `package.el` I see that those errors&messages are
meant to be exposed to the user, so they should preferably use
`error-message-string`.

So I installed the patch below into `master`.
Thank you!
Closing,


        Stefan