fix(express): Incorrest uses of 400 error codes (#1553)

Fixes incorrest usage of 400 HTTP responses being returned from the
server, in favor of using 422.

Also, changed a few return codes to 401 where it was more appropriate.

See this article for reasoning behind moving to 422, and why 400 isn't
appropriate for these cases.

For ref:
6be12f8a06

Related:
https://github.com/meanjs/mean/pull/1547
https://github.com/meanjs/mean/pull/1510

modules/users/server/controllers/users/users.authentication.server.controller.js

@@ -231,7 +231,7 @@ exports.removeOAuthProvider = function (req, res, next) {
 
   user.save(function (err) {
     if (err) {
-      return res.status(400).send({
+      return res.status(422).send({
         message: errorHandler.getErrorMessage(err)
       });
     } else {