input/Plugin: migrate open() from class Error to C++ exceptions

src/lib/ffmpeg/Error.hxx

@@ -20,8 +20,13 @@
 #ifndef MPD_FFMPEG_ERROR_HXX
 #define MPD_FFMPEG_ERROR_HXX
 
+#include <stdexcept>
+
 class Error;
 
+std::runtime_error
+MakeFfmpegError(int errnum);
+
 void
 SetFfmpegError(Error &error, int errnum);