Fix missing error on signin when server has gone away

no issue
- `session.authenticate` calls out to jQuery for the ajax request which then raises it's own error but returns nothing when the server is not reachable. This is a quick fix so that we don't error and can pass through to the default error handler in the authentication step.

core/client/app/controllers/signin.js

@@ -26,7 +26,7 @@ export default Ember.Controller.extend(ValidationEngine, {
             this.get('session').authenticate(authStrategy, model.get('identification'), model.get('password')).catch(function (error) {
                 self.toggleProperty('loggingIn');
 
-                if (error.errors) {
+                if (error && error.errors) {
                     error.errors.forEach(function (err) {
                         err.message = err.message.htmlSafe();
                     });