lib: Prefix GPG errors with the checksum

lib: Prefix GPG errors with the checksum

I was working on https://bugzilla.redhat.com/show_bug.cgi?id=1393545
and it was annoying that I couldn't know what the new (unsigned)
commit has was until verification succeeded.  I could pull it
manually without GPG, but then it'd be sitting in the repo.

Now:

```
Updating from: fedora-atomic:fedora-atomic/25/x86_64/docker-host

Receiving metadata objects: 0/(estimating) -/s 0 bytes
error: Commit 2fb89decd2cb5c3bd73983f0a7b35c7437f23e3aaa91698fab952bb224e46af5: GPG verification enabled, but no signatures found (use gpg-verify=false in remote config to disable)
```

Closes: #663
Approved by: giuseppe