MINOR: spoe: Add status code in error variable instead of hardcoded value

MINOR: spoe: Add status code in error variable instead of hardcoded value

Now, when option "set-on-error" is enabled, we set a status code representing
the error occurred instead of "true". For values under 256, it represents an
error coming from the engine. Below 256, it reports a SPOP error. In this case,
to retrieve the right SPOP status code, you must remove 256 to this value. Here
are possible values:

  * 1:     a timeout occurred during the event processing.
  * 2:     an error was triggered during the ressources allocation.
  * 255:   an unknown error occurred during the event processing.
  * 256+N: a SPOP error occurred during the event processing.