Worker: get download feedback from osbuild

[lavocatt]

Now osbuild gives feedback about failing downloads. Get this info and parse it to inform the user.

Linked to https://github.com/osbuild/osbuild/pull/961

This pull request includes:

• [ ] adequate testing for the new functionality or fixed issue
• [ ] adequate documentation informing people about the change such as
  • [ ] submit a PR for the guides repository if this PR changed any behavior described there: https://www.osbuild.org/guides/