Checkout hangs on incorrect Braintree configuration

[luukveenis]

I ran into an issue today where the checkout in the dummy app would hang indefinitely if the Braintree gateway was misconfigured. Ideally the app should throw an error in this situation.

Steps to reproduce

1. Set up a sandbox account on Braintree
2. Generate and run the dummy application for testing
3. Log in to the admin and create a new payment method using the Braintree gateway
4. Edit the payment method and enter Braintree credentials for your sandbox account
5. Set the environment to development (this is the issue)
6. Checkout as a normal user using the Braintree payment method
7. After selecting Save and Continue on the payment step, the application should hang

If the environment is set to sandbox for the Braintree payment method, the order is processed successfully.