Adds a "canonical" error handling in the module mcf.c,
adhering to the same convention we have adopted in flow.c.
So that when a function fails it signals the caller with an invalid
return value, or bool false, and it communicates an error message.
The main function here is payment_reconsider:
* Each payment has a list of pay_flow.
* This is populated in try_paying(), calling add_payflows & sendpay_new_flows.
* When we get a notification, we resolve a pay_flow using one of the pay_flow_failedxxx
or pay_flow_succeeded functions.
* They call payment_reconsider() which cleans up finished flows decides what to do:
often calling try_paying again.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
