Implements setExceptionMode to allow skipping the exit on exception policy

This is my suggestion to be able to call Stripe.ipn capturing exceptions, so I can call it in a loop contest. If this is accepted here, then this could be done in the Stripe IPN API call.

That would allow automated retries of missed Stripe events, as shown in this snippet.

Thanks to @jamie for making me realize I sent yesterday this same pull request to a fork, instead of here.