Add option to scrobbler setting for showing login error

Fixes #430

src/settings/scrobblersettingspage.cpp

@@ -94,6 +94,7 @@ void ScrobblerSettingsPage::Load() {
   ui_->checkbox_offline->setChecked(scrobbler_->IsOffline());
   ui_->spinbox_submit->setValue(scrobbler_->SubmitDelay());
   ui_->checkbox_albumartist->setChecked(scrobbler_->PreferAlbumArtist());
+  ui_->checkbox_show_auth_error->setChecked(scrobbler_->ShowAuthError());
 
   ui_->checkbox_lastfm_enable->setChecked(lastfmscrobbler_->IsEnabled());
   ui_->checkbox_lastfm_https->setChecked(lastfmscrobbler_->IsUseHTTPS());
@@ -119,6 +120,7 @@ void ScrobblerSettingsPage::Save() {
   s.setValue("offline", ui_->checkbox_offline->isChecked());
   s.setValue("submit", ui_->spinbox_submit->value());
   s.setValue("albumartist", ui_->checkbox_albumartist->isChecked());
+  s.setValue("show_auth_error", ui_->checkbox_show_auth_error->isChecked());
   s.endGroup();
 
   s.beginGroup(LastFMScrobbler::kSettingsGroup);

src/settings/scrobblersettingspage.ui

@@ -109,6 +109,13 @@
      </property>
     </widget>
    </item>
+   <item>
+    <widget class="QCheckBox" name="checkbox_show_auth_error">
+     <property name="text">
+      <string>Show error dialog if scrobbler is enabled without being authenticated</string>
+     </property>
+    </widget>
+   </item>
    <item>
     <widget class="QGroupBox" name="groupbox_lastfm">
      <property name="title">