Improve page for nonexistent keywords

[bluss]

Trying an URL to see if a keyword exists gives an ugly page: https://crates.io/keywords/nosuch

It could be improved to just say they keyword does not exist.

Ember Data Request GET /api/v1/keywords/nosuch returned a 404 Payload (application/json; charset=utf-8) [object Object]

  r@https://crates.io/assets/vendor-b5eaa22629fc0ae42aba95c5979b7b1f.js:9:30041
i@https://crates.io/assets/vendor-b5eaa22629fc0ae42aba95c5979b7b1f.js:24:8997
d<.handleResponse@https://crates.io/assets/vendor-b5eaa22629fc0ae42aba95c5979b7b1f.js:24:14922
d<.ajax/</c.error@https://crates.io/assets/vendor-b5eaa22629fc0ae42aba95c5979b7b1f.js:24:15477
ie.Callbacks/c@https://crates.io/assets/vendor-b5eaa22629fc0ae42aba95c5979b7b1f.js:3:26369
ie.Callbacks/d.fireWith@https://crates.io/assets/vendor-b5eaa22629fc0ae42aba95c5979b7b1f.js:3:27191
r@https://crates.io/assets/vendor-b5eaa22629fc0ae42aba95c5979b7b1f.js:5:9124
.send/t@https://crates.io/assets/vendor-b5eaa22629fc0ae42aba95c5979b7b1f.js:5:15309

[alexcrichton]

Fixed by #445