Error instead of blocking on (some) incorrect usage

[achalddave]

I was trying to parse a large file, and was very surprised to find that underscore was taking so long to return results on the file. It turns out underscore was actually not doing anything, since I hadn't written the command correctly. Specifically:

underscore keys [filename]

just blocks without any errors, while

underscore keys <filename

is the correct incantation. It would be nice if the former reported an error...