Add-on store: Notify user when download/fetch fails (#14846)
Part of #13985
Summary of the issue:
The add-on store may fail to fetch the latest data.
The add-on store may also have a problem downloading an add-on.
Currently, users are not notified of these failures.
Description of user facing changes
Users are now notified when an add-on fails to download, or when the add-on store data fails to be fetched.
Description of development approach
A "DisplayError" exception class was created based on the "TranslatedError" used previously.
When raising this exception, translatable strings for an error message are required.
When handling this exception, a message modal is created displaying the warning.
This allows injecting a error display handler for the add-on data-manager.
This is important if external components use the add-on data-manager, other than the add-on store GUI.
For example, when there is an automatic add-on updater.
