perl/lua/python destinations: handle errors

[faxm0dem]

It would be awesome if we could have some way to handle errors.
For instance for perl, we could have new callbacks on_error which would fire up in case of a non-true return value, with the callback function as argument. By default this callback could be to throw an error. Then syslog-ng could restart the process after a configurable time. Of course if flow-control is on it would be nice not to lose messages during that time.