change 403 error with 409 in adduser

Because npm does only accept 409, see #184.

lib/index-api.js

@@ -136,10 +136,11 @@ module.exports = function(config, auth, storage) {
       }
       auth.add_user(req.body.name, req.body.password, function(err, user) {
         if (err) {
-          if (err.status < 500 && err.message === 'this user already exists') {
-            // with npm registering is the same as logging in
-            // so we replace message in case of conflict
-            return next( Error[409]('bad username/password, access denied') )
+          if (err.status >= 400 && err.status < 500) {
+            // With npm registering is the same as logging in,
+            // and npm accepts only an 409 error.
+            // So, changing status code here.
+            return next( Error[409](err.message) )
           }
           return next(err)
         }