Stripe requests blocked by browser extensions

[Changaco]

We used to detect third-party requests being blocked by a browser extension during payments, however this was “lost” when we moved from Mangopay to Stripe. The lack of any in-page error message is confusing for the user, e.g. #1637.

To fix this issue, js/stripe.js needs to be modified to somehow detect the failure to load the Stripe resources and insert a warning message for the user into the page.

[Quix0r]

Or avoid it at all.