dnf5 copr: missing error handling for Copr-Error-Data header

[nikromen]

follow up from #3584

On any http error retval, the dnf5 copr does nothing - no warning, no output, it just quietly ends since there's missing handling of the Copr-Error-Data header

[root@b82250a064c2 /]# dnf copr enable mblaha/coprtest -y epel-9-aarch64
 https://copr.fedorainfracloud.org/api_3/rpmrepo/mblaha/coprtest/fedora-41/                         100% | 320.0   B/s | 276.0   B |  00m01s
Enabling a Copr repository. Please note that this repository is not part
of the main distribution, and quality may vary.

The Fedora Project does not exercise any power over the contents of
this repository beyond the rules outlined in the Copr FAQ at
<https://docs.pagure.org/copr.copr/user_documentation.html#what-i-can-build-in-copr>,
and packages are not held to any quality or security level.

Please do not file bug reports about these packages in Fedora
Bugzilla. In case of problems, contact the owner of this repository.
[root@b82250a064c2 /]#