Add feedback on login errors

There was no feedback on any login errors when a user provided a
wrong url, username, password or a disabled API. This commit
adds feedback to the user in this cases.

Fixes #15

qml/main.qml

@@ -21,6 +21,7 @@ import QtQuick 2.3
 import QtQuick.Controls 1.3
 import QtQuick.Window 2.0
 import QtQuick.Layouts 1.1
+import QtQuick.Dialogs 1.1
 import Qt.labs.settings 1.0
 import TTRSS 1.0
 
@@ -210,14 +211,27 @@ ApplicationWindow {
         visible: !serverLogin.loggedIn()
 
         function login() {
-            console.log("FOO")
             serverLogin.login(serverUrl, userName, password)
         }
+
+    }
+
+    MessageDialog {
+        id: loginErrorAlert
+        title: "A login error occured"
+        text: serverLogin.loginError
+        onAccepted: visible = false
     }
 
     ServerLogin {
         id: serverLogin
         onSessionIdChanged: app.loggedIn()
+        onLoginErrorChanged: {
+            console.log("loginError:", loginError)
+            if(loginError.length > 0) {
+                loginErrorAlert.visible = true
+            }
+        }
     }
 
     Server {