playlistparser: Display errors on failures.

Add an Error signal to PlayListParser and connect that to the
PlayListManager::Error signal. This is eventually connected to the
Application::AddError. Add error messages in some cases where
PlayListParser can fail silently.

src/playlist/playlistmanager.cpp

@@ -79,6 +79,7 @@ void PlaylistManager::Init(LibraryBackend* library_backend,
   connect(library_backend_, SIGNAL(SongsRatingChanged(SongList)),
           SLOT(SongsDiscovered(SongList)));
 
+  connect(parser_, SIGNAL(Error(QString)), this, SIGNAL(Error(QString)));
   for (const PlaylistBackend::Playlist& p :
        playlist_backend->GetAllOpenPlaylists()) {
     AddPlaylist(p.id, p.name, p.special_type, p.ui_path, p.favorite);

src/playlistparsers/playlistparser.cpp

@@ -129,6 +129,7 @@ SongList PlaylistParser::LoadFromFile(const QString& filename) const {
   // Find a parser that supports this file extension
   ParserBase* parser = ParserForExtension(info.suffix());
   if (!parser) {
+    emit Error(tr("Unknown filetype: %1").arg(filename));
     qLog(Warning) << "Unknown filetype:" << filename;
     return SongList();
   }
@@ -159,6 +160,7 @@ void PlaylistParser::Save(const SongList& songs, const QString& filename,
   // Find a parser that supports this file extension
   ParserBase* parser = ParserForExtension(info.suffix());
   if (!parser) {
+    emit Error(tr("Unknown filetype: %1").arg(filename));
     qLog(Warning) << "Unknown filetype:" << filename;
     return;
   }

src/playlistparsers/playlistparser.h

@@ -55,6 +55,9 @@ class PlaylistParser : public QObject {
   void Save(const SongList& songs, const QString& filename,
             Playlist::Path) const;
 
+ signals:
+  void Error(const QString& msg) const;
+
  private:
   QString FilterForParser(const ParserBase* parser,
                           QStringList* all_extensions = nullptr) const;