add-on store: Handle bad download error (#15861)
Fixes #15796
Summary of the issue:
Sometimes an add-on file fails to download correctly.
This causes an error that isn't recoverable without notification to the user.
Description of user facing changes
If an add-on download file cannot be successfully loaded as a zipfile, warn the user that the installation has failed and let the user continue with their tasks.
in my opinion this fix is not worthy of a changelog entry
Description of development approach
Catch error
