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.
Avoids a gratuitous "ctx" field, and the simplified declaration
is now understood by `make update-mocks`.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
