MINOR: sample: make smp_resolve_args() return an allocate error message

MINOR: sample: make smp_resolve_args() return an allocate error message

For now smp_resolve_args() complains on stderr via ha_alert(), but if we
want to make it a bit more dynamic, we need it to return errors in an
allocated message. Let's pass it an error pointer and have it fill it.
On return we indent the output if it contains more than one line.