Keep track of erros while opening a comic

This way we can query if an error ocurred opening a comic.

common/comic.h

@@ -40,6 +40,8 @@ class Comic : public QObject
 
         bool _invalidated;
 
+        bool _errorOpening;
+
 	public:
 		
 		static const QStringList imageExtensions;
@@ -73,6 +75,9 @@ class Comic : public QObject
 		QByteArray getRawPage(int page);
 		bool pageIsLoaded(int page);
 
+        //check if the comic has failed loading
+        bool hasBeenAnErrorOpening();
+
 		inline static QStringList getSupportedImageFormats() { return imageExtensions;}
 		inline static QStringList getSupportedImageLiteralFormats() { return literalImageExtensions;}