Slightly better error message if you type ngrok with no args

I wanted to do os.isatty for Go, to make it red if the user's printing to the terminal, but the closest I could find was http://godoc.org/code.google.com/p/go.crypto/ssh/terminal#IsTerminal and that was still reporting True if I redirected stdout to a file, so not sure whether that would work that well.

Anyway, this is an improvement on what is there currently