boot-deploy silently fails on non-existent dtb names

Discovered by @LogicalErzor in https://gitlab.com/postmarketOS/pmaports/-/merge_requests/4205#note_1455931316

It seems that boot-deploy no longer gives an error if it doesn't find a dtb, but instead just doesn't append it. This is very confusing behaviour, and is a regression. I've not bisected it, but both the currently packaged version in pmOS, 0.8.2, and the latest tagged version, 0.9.1, exhibit this issue.

To reproduce this, take an arbitrary device package that uses deviceinfo_dtb, and set it to some obviously non-existent value. boot-deploy will not report any errors.