Hi @ZSpl,
I don’t think HubSpot provides a way to get notified in case of notification failure for subscription-based webhooks.
However, HubSpot does part of the error handling and retries for up to 10 times within a 24h period as mentionned in the documentation.
@TZiesing, to get additionnal properties to reuse after executing your webhook you’ll have to test it out :
If you test it, and you have set up your server to respond with additionnal fields, you’ll be able to select them as the output of the webhook action.
Hope this helps !
If it does, please consider marking this answer as a solution 
Best,
Ludwig