Add-on store: Clear download cache where appropriate (#15471)
Fixes #15469
Summary of the issue:
Cancelled/failed downloads from the add-on store are not cleared.
This prevents future attempts at installation.
Description of user facing changes
Users should now be able to install add-ons after cancelling or if the download fails
Description of development approach
Clear the download cache:
when an add-on download fails/cancels
when exiting the store
when opening the store
