Add-on store: Clean up failed installs (#15921)
Fixes #15719
Summary of the issue:
If an add-ons install failed, it might never be cleaned up, causing NVDA to be constantly expecting there are add-ons pending install.
This causes a warning dialog that NVDA must be restarted when closing the add-on store.
Description of user facing changes
Pending installs that fail are removed and cleaned up, ensuring it is easy to attempt a future install.
The warning dialog no longer fires as NVDA cleans up failed installs.
Description of development approach
Clear the pending install set and delete pending install add-on folders after loading add-ons.
When loading add-ons, all installs should complete, and there should be no new pending installs from the user.
If an install fails, also attempt to clean up the files immediately after.
