handle idempotence key error (#1200)
handle idempotence key error (#1200)

reading from rand can return an error and fail to read bytes, this
leads to the second component not being random.

This solution panics in that case, similar to getting a new uuid
https://github.com/google/uuid/blob/master/version4.go#L13-L15

the side effect is, in the case the second component is not random (an
error happened) we are now panicing instead of continuing

Signed-off-by: Andrew Klotz <e02bb6f28f3c7ee950c3eb5799e9beb0a21495e7@gmail.com>
stripe/stripe-go,stripe/stripe-go