Argument validation should be moved out from the argument parsers

[akamsteeg]

Currently, it's the task of the ICommandLineArgumentsParser implementations to validate the arguments using the supplied ArgumentValidator instances. Since this is the same for all parsers (and it's hard to get right) we should move it out to the call site.

[akamsteeg]

Hm, we might be able to shoehorn in a system for parse errors too. Now we silently disregard those, but that's not ideal.

[akamsteeg]

This is actually quite hard to do, so I'm moving this to "somewhere in the future".