addons-frontend: Error _promiseRejectionHandler in Sentry

https://sentry.prod.mozaws.net/operations/addons-frontend-amo-prod/issues/6074703/

Error: 
  at _promiseRejectionHandler (/en-US/firefox/addon/ghostery/)

For QA: It is possible to reproduce by:

1. loading a product page for a webextension
2. clicking the install button
3. cancelling the install process

With these STRs, we get the error in the console.

Such issues are not new apparently (first seen in 2019.07.18) but we see more than before it seems. It could be because most people read the “warning” message and cancel the installation, or something else…