Error on running cron job for mailing process on CivicRM 4.7.8
Check that:


your command line specifies the -s parameter, as it is used by CiviMail (cf. https://wiki.civicrm.org/confluence/display/CRMDOC/Managing+Scheduled+Jobs#ManagingScheduledJobs-PHPclimethod)
the user/password pair on the command line is for a valid Joomla user (log into the website with this exact username and password)
that this user has at least the Access CiviCRM and Administer CiviCRM permissions in Joomla