Improve "Payment failed" error messages

[vdavid]

They all look like this now:

For each of these card types: https://docs.stripe.com/testing#declined-payments and others on the page, we want to provide informative error messages instead of "We couldn't create the Stripe token. This is what happened: Error: Request to Cody Pro API failed: Bad Gateway".

[vdavid]

Oh, actually, it was my mistake, my DB was in the wrong state. We actually display the failure-specific error messages:

The problem is that in case of a payment failure, we create the user and team, but fail to create the subscription and don't roll back the user+team creation steps, so the DB ends up in a broken state.
Created issue for that: https://github.com/sourcegraph/self-serve-cody/issues/877.

I'll still leave the issue open, though, because the error message could be clearer and more in line with the rest of the design.

[vdavid]

They get even nastier: