Various changes for #157

Add retryable and error_code fields to PaymentFailed event
Add RouteUpdate event ala #217 containing msgs::HTLCFailChannelUpdate
Move msgs::HTLCFailChannelUpdate from update_fail_htlc's return value to
the payload of RouteUpdate fired inside fail_htlc_backwards_interanl
Some fixes in decode_update_add_htlc_onion
Move procesing onion failure to fail_htlc_backwards_internal
Test for process_onion_failure

Author: yuntai