We were already adding origin IDs to error messages; this extends the
same mechanism to allow adding app IDs as well.

The motivation for doing so is to be able to change the flatpak plugin
from updating one app at a time to doing all apps in one transaction,
but when we do that then the plugin loader no longer knows how to map
errors to apps. This adds the mechanism for that.

It's all pretty hacky, but not more so than the existing origin ID
passing.

This also improves things somewhat and removes the need for the UI code
to know and handle unique ID prefixes in the error messages. Instead,
the plugin loader now does all the work and strips any unique ID
prefixes.