better error message for 404

[dominictarr]

currently you get this if a module name does not exist:
not obvious.

 npmd install level-binomial-replicate
GET https://registry.npmjs.org/level-binomial-replicate
404 https://registry.npmjs.org/level-binomial-replicate

/home/dominic/.nave/installed/0.10.26/lib/node_modules/npmd/node_modules/npmd-resolve/online.js:101
      var versions = Object.keys(json.versions)
                            ^
TypeError: Object.keys called on non-object
    at Function.keys (native)
    at next (/home/dominic/.nave/installed/0.10.26/lib/node_modules/npmd/node_modules/npmd-resolve/online.js:101:29)
    at /home/dominic/.nave/installed/0.10.26/lib/node_modules/npmd/node_modules/npmd-resolve/online.js:75:13
    at Object.oncomplete (fs.js:107:15)