This causes a compiler warning if we don't do something with the
result (hopefully return immediately!).
We use was_pending() to ignore the result in the case where we
complete a command in a callback (thus really do want to ignore
the result).
This actually fixes one bug: we didn't return after command_fail
in json_getroute with a bad seed value.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>