[metatools] Add minimal error handling to merge-kits so that the broken package name is clear

[cuantar]

The autogen (a custom autogen.py that generates multiple branches) currently is broken:
ERROR Upstream GitHub asset binaryen-version_120-aarch64-linux.tar.gz not found.

The real issue here is that merge-kits does not give helpful error messages when an autogen causes tree regen to abort. At the very least, it should report the name of the package that broke it.

[cuantar]

I can't confirm this in my dev environment, so perhaps MongoDB is not the culprit that is breaking the tree.

It seems fine in both next and mark-testing releases.

[cuantar]

I've changed this issue so that the motivation for the metatools feature it now suggests is clear.