DZT command line should properly error out/validate params that are passed in

[mzikherman]

The second param passed in is actually invalid, and it seems to just get ignored.

➜  gravity git:(master) ✗ dzt slice ~/Desktop/skull.jpg /tmp/tiles
/Users/mzikherman/Artsy/source/gravity/tiles/11
/Users/mzikherman/Artsy/source/gravity/tiles/10
/Users/mzikherman/Artsy/source/gravity/tiles/9
/Users/mzikherman/Artsy/source/gravity/tiles/8
/Users/mzikherman/Artsy/source/gravity/tiles/7
/Users/mzikherman/Artsy/source/gravity/tiles/6
/Users/mzikherman/Artsy/source/gravity/tiles/5
/Users/mzikherman/Artsy/source/gravity/tiles/4
/Users/mzikherman/Artsy/source/gravity/tiles/3
/Users/mzikherman/Artsy/source/gravity/tiles/2
/Users/mzikherman/Artsy/source/gravity/tiles/1
/Users/mzikherman/Artsy/source/gravity/tiles/0