Parse Stripe error correctly.

Sources/StripeKit/Errors/StripeError.swift

@@ -11,6 +11,10 @@ import Foundation
 /// Stripe uses conventional HTTP response codes to indicate the success or failure of an API request. In general: Codes in the `2xx` range indicate success. Codes in the `4xx` range indicate an error that failed given the information provided (e.g., a required parameter was omitted, a charge failed, etc.). Codes in the `5xx` range indicate an error with Stripe's servers (these are rare).
 /// Some `4xx` errors that could be handled programmatically (e.g., a card is declined) include an error code that briefly explains the error reported.
 public final class StripeError: StripeModel, Error {
+    public var error: _StripeError?
+}
+
+public final class _StripeError: StripeModel {
     /// The type of error returned. One of `api_connection_error`, `api_error`, `authentication_error`, `card_error`, `idempotency_error`, `invalid_request_error`, or `rate_limit_error`
     public var type: StripeErrorType?
     /// For card errors, the ID of the failed charge.