Web: Fix tapping outside alert/confirm dialogs doesn't dismiss them correctly

[personalizedrefrigerator]

Summary

This pull request fixes an issue where calls to shim.showMessageBox could return a promise that never resolves if the dialog was dismissed by tapping on its background. This issue was present on web and possibly Android.

Testing plan

In addition to this pull request's automated test, the change has been tested manually by:

1. Opening the plugin configuration screen.
2. Installing an invalid plugin from a file.
3. Waiting for the error dialog to be shown.
4. Tapping outside of the dialog to dismiss it.
5. Verifying that the "Installing" button no longer shows a "loading" indicator.

[laurent22]

There's a conflict on one file