[backend] add debug output for preinstallimage miss

[adrianschroeter]

Unfortunatly only the md5sums, one need to grep the preinstallimage.info
file for package names.

Replace this line with a description of your changes. Please follow the suggestions in the comment below.

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If this PR requires any particular action or consideration before deployment,
please check the reasons or add your own to the list:

• Requires a database migration that can cause downtime. Postpone deployment until maintenance window[1].
• Contains a data migration that can cause temporary inconsistency, so should be run at a specific point of time.
• Changes some configuration files (e.g. options.yml), so the changes have to be applied manually in the reference server.
• A new Feature Toggle[2] should be enabled in the reference server.
• Proper documentation or announcement has to be published upfront since the introduced changes can confuse the users.

[1] https://github.com/openSUSE/open-build-service/wiki/Deployment-of-build.opensuse.org#when-there-are-migrations
[2] https://github.com/openSUSE/open-build-service/wiki/Feature-Toggles-%28Flipper%29#you-want-real-people-to-test-your-feature