LGTM, but would prefer if Package.formatError(/s?) would format all the errors by instead of just picking up the first one.

In this case, I'm not quite sure what the UI should look like.
At this point, we have a list of * filename: error message (we need it be a list as in update/uninstall scenarios, several extensions can fail at once).
I think it would look odd if one extension was listed several times.

I would be okay with just them being separated with a dot, as long as the information doesn't go missing.

That works, I guess.
Looks like this now:

Also added the MISSING_PACKAGE_JSON string because we didn't have one before.

LGTM, merging this.