playlistparser: Add error mechanism for parsers.

Add a new Error signal to parser implementations. Display an error when
user tries to use the unimplemented CueParser::Save.

src/playlistparsers/cueparser.cpp

@@ -348,7 +348,9 @@ qint64 CueParser::IndexToMarker(const QString& index) const {
 
 void CueParser::Save(const SongList& songs, QIODevice* device, const QDir& dir,
                      Playlist::Path path_type) const {
-  // TODO
+  // TODO: Not yet implemented. Cue files represent tracks within a single
+  //       file, so the song list would need to be composed properly.
+  emit Error(tr("Saving cue files is not yet supported."));
 }
 
 // Looks for a track starting with one of the .cue's keywords.

src/playlistparsers/parserbase.h

@@ -55,6 +55,9 @@ class ParserBase : public QObject {
       const SongList& songs, QIODevice* device, const QDir& dir = QDir(),
       Playlist::Path path_type = Playlist::Path_Automatic) const = 0;
 
+ signals:
+  void Error(const QString& msg) const;
+
  protected:
   // Loads a song.  If filename_or_url is a URL (with a scheme other than
   // "file") then it is set on the song and the song marked as a stream.

src/playlistparsers/playlistparser.cpp

@@ -35,13 +35,18 @@ PlaylistParser::PlaylistParser(LibraryBackendInterface* library,
                                QObject* parent)
     : QObject(parent) {
   default_parser_ = new XSPFParser(library, this);
-  parsers_ << new M3UParser(library, this);
-  parsers_ << default_parser_;
-  parsers_ << new PLSParser(library, this);
-  parsers_ << new ASXParser(library, this);
-  parsers_ << new AsxIniParser(library, this);
-  parsers_ << new CueParser(library, this);
-  parsers_ << new WplParser(library, this);
+  AddParser(new M3UParser(library, this));
+  AddParser(default_parser_);
+  AddParser(new PLSParser(library, this));
+  AddParser(new ASXParser(library, this));
+  AddParser(new AsxIniParser(library, this));
+  AddParser(new CueParser(library, this));
+  AddParser(new WplParser(library, this));
+}
+
+void PlaylistParser::AddParser(ParserBase* parser) {
+  parsers_ << parser;
+  connect(parser, SIGNAL(Error(QString)), this, SIGNAL(Error(QString)));
 }
 
 QStringList PlaylistParser::file_extensions() const {

src/playlistparsers/playlistparser.h

@@ -59,6 +59,8 @@ class PlaylistParser : public QObject {
   void Error(const QString& msg) const;
 
  private:
+  void AddParser(ParserBase* parser);
+
   QString FilterForParser(const ParserBase* parser,
                           QStringList* all_extensions = nullptr) const;