When failing to pull an image from a secured registry we should be more explicit and add possible solution on error.

[jmarrero]

From the CoreOS matrix channel:

ostree native containers: i'm experimenting with this again but struggling. 
i can rebase to the standard quay.io fcos images. and when i push one of
these to my own docker.io private registry i can successfully sudo podman pull said image. 
but if I try to rebase i'm failing on permissions. some strange error like 

error: Creating importer: Failed to invoke skopeo proxy method OpenImage: 
remote error: reading manifest testing in docker.io/someuser/someimage: 
requested access to the resource is denied.

Any hints?(edited)

From error: Creating importer: Failed
I imagine we could error check on:
https://github.com/ostreedev/ostree-rs-ext/blob/main/lib/src/cli.rs#L641
if I understand correctly...