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"

lib/index.js

@@ -64,7 +64,15 @@ module.exports = function(config_hash) {
 			if (err.status && err.status >= 400 && err.status < 600) {
 				if (calls == 1) {
 					res.status(err.status)
-					res.send({error: err.msg || err.message || 'unknown error'})
+					var body = {error: err.msg || err.message || 'unknown error'};
+
+					// Make 404 responses compliant with CouchDB REST API
+					if (err.status == 404) {
+						body.reason = body.error
+						body.error = 'not_found'
+					}
+
+					res.send(body)
 				}
 			} else {
 				Logger.logger.error({err: err}, 'unexpected error: @{!err.message}\n@{err.stack}')