Improve UI/UX when users try to add an invalid card with Stripe

Summary: Ref T13244. See PHI1052. Our error handling for Stripe errors isn't great right now. We can give users a bit more information, and a less jarring UI.

Test Plan:
Before (this is in developer mode, production doesn't get a stack trace):

After:

• Tried all the invalid test codes listed here: https://stripe.com/docs/testing#cards

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13244

Differential Revision: https://secure.phabricator.com/D20132