Don't attempt to refund an order with an invalid amount

[reykjalin]

When the order amount is invalid (e.g. when you try to refund an amount that's 0) we still try to process a refund through Stripe. We should catch the issue on the API level and inform the merchant that the refund amount is invalid — without sending the refund request to the Stripe API.

If you just click the refund button (without a valid amount) you get an alert with the error "Invalid refund amount"

Originally reported by @diegocurbelo in #2167 (comment)

Reproduction steps

1. Pay for an order with 4242 4242 4242 4242
2. Go to the order details screen
3. Click Refund
4. Try to refund an invalid amount via WCPay, e.g. 0, negative amounts, and amounts greater than the order's cost.