Improve error handling for Stripe payment session

[dd32]

This resolves a fatal encountered, and some php warnings.

• $stripe->get_session: When this timed out (30s) WordCamp fatal error'd instead of gracefully handling it.
  • This was not a purchase, as it occured after the event, My best guess is someone clicked a link in a purchase log.
  • This error condition be better be picked up via Payment Methods: Harden validation of payments #1551
• payment_intent: This is always set with the WordCamp default credentials, but isn't with a specific local event teams Stripe account.. It's not clear why..