Should exit with error status on invalid arguments

[acoulton]

If running a task with invalid arguments, minion currently just prints the validation error to stdout and skips execution of the task, eventually terminating with an exit code of 0.

I would expect it to quit with a nonzero code in this case, since the task has failed.

There's 3 options for fixes:

• return an exit code from Minion_Task::execute and pass it to exit in the script wrapper. This would mean modifying index.php in kohana/kohana and requiring all users to change their own projects to match.
• throw an exception from Minion_Task::execute and assume it will be unhandled and cause php to quit with an exit code.
• just exit from Minion_Task::execute - hard to test and generally nasty.

Returning an exit code feels cleanest, but I think throwing an exception is the most sensible bugfix at this point?