Move argv check to ShellDispatcher.

This is closer to the actual problem. The error message will look something like this
```
[24-Apr-2014 09:35:03 UTC] PHP Notice:  Undefined variable: argv in ***/app/Console/cake.php on line 35
[24-Apr-2014 09:35:03 UTC] PHP Fatal error:  Uncaught exception 'Exception' with message '$argv must be an array. Check CLI configuration.' in ***/lib/Cake/Console/ShellDispatcher.php:66
Stack trace:
#0 ***/app/Console/cake.php(35): ShellDispatcher::run(NULL)
#1 {main}
  thrown in ***/lib/Cake/Console/ShellDispatcher.php on line 66
```