Make 404 responses compatible with CouchDB API

The CouchDB REST API returns always `"error": "not_found"` in the body
of a 404 response:
  http://couchdb-13.readthedocs.org/en/latest/api-basics/#http-status-codes

The npm client depends on the magic string 'not_found' as can be seen
in requestDone() in npm-registry-client/lib/request.js.

Before this change, npm install of an unknown package was reporting
the Sinopia error string and a stack trace of npm.

After this change, npm install of an unknown package returns a nice
error saying "the package is not in the npm registry, bug the author"
Merge pull request #56 from strongloop/fix-unknown-package-name-response

Make 404 responses compatible with CouchDB API