Fix device loader error messages. (#6215)

GPodLoader and MtpLoader Error signals were connected to Error signals in their repective device classes, but the actual signal definition in ConnectedDevice was removed in a refactor several years ago. This change adds LoaderError slots to these device classes and reports the error in the manner of the refactored code.

src/devices/gpoddevice.cpp

@@ -47,7 +47,7 @@ void GPodDevice::Init() {
                            shared_from_this());
   loader_->moveToThread(loader_thread_);
 
-  connect(loader_, SIGNAL(Error(QString)), SIGNAL(Error(QString)));
+  connect(loader_, SIGNAL(Error(QString)), SLOT(LoaderError(QString)));
   connect(loader_, SIGNAL(TaskStarted(int)), SIGNAL(TaskStarted(int)));
   connect(loader_, SIGNAL(LoadFinished(Itdb_iTunesDB*)),
           SLOT(LoadFinished(Itdb_iTunesDB*)));
@@ -223,6 +223,10 @@ void GPodDevice::FinishDelete(bool success) {
   ConnectedDevice::FinishDelete(success);
 }
 
+void GPodDevice::LoaderError(const QString& message) {
+  app_->AddError(message);
+}
+
 bool GPodDevice::GetSupportedFiletypes(QList<Song::FileType>* ret) {
   *ret << Song::Type_Mp4;
   *ret << Song::Type_Mpeg;

src/devices/gpoddevice.h

@@ -53,6 +53,7 @@ class GPodDevice : public ConnectedDevice, public virtual MusicStorage {
 
  protected slots:
   void LoadFinished(Itdb_iTunesDB* db);
+  void LoaderError(const QString& message);
 
  protected:
   Itdb_Track* AddTrackToITunesDb(const Song& metadata);

src/devices/mtpdevice.cpp

@@ -54,7 +54,7 @@ void MtpDevice::Init() {
       new MtpLoader(url_, app_->task_manager(), backend_, shared_from_this());
   loader_->moveToThread(loader_thread_);
 
-  connect(loader_, SIGNAL(Error(QString)), SIGNAL(Error(QString)));
+  connect(loader_, SIGNAL(Error(QString)), SLOT(LoaderError(QString)));
   connect(loader_, SIGNAL(TaskStarted(int)), SIGNAL(TaskStarted(int)));
   connect(loader_, SIGNAL(LoadFinished()), SLOT(LoadFinished()));
   connect(loader_thread_, SIGNAL(started()), loader_, SLOT(LoadDatabase()));
@@ -69,6 +69,8 @@ void MtpDevice::LoadFinished() {
   db_busy_.unlock();
 }
 
+void MtpDevice::LoaderError(const QString& message) { app_->AddError(message); }
+
 bool MtpDevice::StartCopy(QList<Song::FileType>* supported_types) {
   // Ensure only one "organise files" can be active at any one time
   db_busy_.lock();

src/devices/mtpdevice.h

@@ -60,6 +60,7 @@ class MtpDevice : public ConnectedDevice {
 
  private slots:
   void LoadFinished();
+  void LoaderError(const QString& message);
 
  private:
   bool GetSupportedFiletypes(QList<Song::FileType>* ret,