Fetches the error message when manifest upload fails.

Uploading manifest seems to fail frequently when uploading at the
jenkins end. This will help us know the exact error.
As the automated installation had failed, downloading and placing the fake_manifest.crt too failed and there was no way candlepin could verify the signature for the manifest file.

Now with the above error, it quickly helped me identify that there was no fake_manifest.crt at `/etc/candlepin/certs/upstream`.