Add fallback incase stripe_customer_id is not valid

Because the module no longer creates a new stripe customer for every order, there is a slight chance that a drupal user's saved stripe customer ID is not valid in stripe.

Scenarios:
Making an order in stripe test mode and then live mode using the same drupal user.
If you change to a different stripe account for your payments.
The customer has been deleted from stripe.

Fix:
Do a check that the saved customer id is valid. If it's invalid based on stripe, just create a new stripe customer and save that ID.