Properly handle API errors with unknown error types

[praboud-stripe]

When the API returns an error of a type which isn't explicitly enumerated, StripeError.generate tries to return a GenericError. However, that class doesn't exist - it was deleted as part of the refactor in #672. This causes stripe-node to return an especially confusing error - it complains about the undefined GenericError class, instead of telling us anything useful about the actual error.

This patch defines a fallback subclass of StripeError to use in these cases. (As a bonus, this means that the default error rendering will also contain the error message from the API, instead of a fixed Unknown Error string.)

AFAICT, the only way it was possible to hit this branch is to get the oauth flow to return weird errors. Since this is served from connect.stripe.com instead of api.stripe.com, it can return non-standard error types. It's not clear to me whether it's worth creating new error classes for these cases - for now, I've just fixed the type error.

[CLAassistant]

All committers have signed the CLA.

[richardm-stripe]

Thank you @praboud-stripe, this looks good to me!