Improve EOL handling

[voxik]

Koschei recently stopped to process packages [1]. This was later identified to be due to F35 EOL. While I have filled this [2] ticket and hopefully relengs will handle Koschei when Fedora release goes EOL, I think that Koschei could be also made more robust and keep churning even if it cannot access some information about specific Fedora release.

[mizdebsk]

First of all, Koschei strives to be agnostic of particular distribution it is used for. It does not have have knowledge of Fedora release schedule. Koschei admins configure it to use particular Koji tags and build targets.

The linked issue happened because Koji build target was deleted and Koschei configuration was not updated accordingly. Koschei kept trying to submit scratch build for f35 build target that no longer existed, which ended in Koji XML-RPC fault. Scratch builds that fail due to Koji fault are ignored, see #42. This way scratch builds for other build targets were not submitted as they were pre-emptied by higher-priority f35 builds.

Ideas of what could be improved on Koschei side is to improve detection of transient Koji faults - non-existent Koji build target is not a transient fault and should not be ignored.