Make emails unique

Emails are made unique. When user attempts to sign in through a provider in which his email is one that is already registered, user is redirected to the signin page with an error passed as a query string parameter.

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

@@ -83,12 +83,15 @@ exports.signout = function (req, res) {
 exports.oauthCallback = function (strategy) {
 	return function (req, res, next) {
 		passport.authenticate(strategy, function (err, user, redirectURL) {
-			if (err || !user) {
-				return res.redirect('/#!/signin');
+			if (err) {
+				return res.redirect('/authentication/signin?err=' + encodeURIComponent(errorHandler.getErrorMessage(err)));
+			}
+			if (!user) {
+				return res.redirect('/authentication/signin');
 			}
 			req.login(user, function (err) {
 				if (err) {
-					return res.redirect('/#!/signin');
+					return res.redirect('/authentication/signin');
 				}
 
 				return res.redirect(redirectURL || '/');