flatpak: Don't disable the plugin if an installation setup fails
When setting up the Flatpak plugin, if a Flatpak Installation had a problem (corrupt repository's config, etc.), then the plugin got disabled and just logged the error. This is a problem because:
- The user is not informed that an error occurred;
- The user won't be able to use e.g. the system installation even if only the user's installation failed.
This patch changes that so that an error event (in-app banner) is shown to the user, and allows the plugin to continue enabled unless all of the installations have failed.
Originally by @jrocha; tidied up and commit message slightly reworded by me.