Issue 6743 - Print error if -run is followed by a path with an incompatible extension

http://d.puremagic.com/issues/show_bug.cgi?id=6743

Extensionless files are OK, this is a DMD feature, we only have to check when there's an extension. I'm allowing `.di` files because this already worked before, there's no need to break this feature even though it might be uncommon to run `.di` files.