Fix VODieR script error on startup in Frodo

Hey Liam,

Just a small tweak to fix a script error on startup in Frodo (I'm using RC3).

This appears to be caused in the ParseArgs function where sys.argv[2] is true but does not contain 'mode=' in the string that is passed from argv.  

This should fix the issue raised in https://github.com/liamf/VODieR/issues/16

There's no doubt a better way of fixing this, but this seems the simplest solution in the interim.

Cheers!
Nick