[Subtitles] Change error message to warning message when fail subtype detection

Because this is really boring when you have some .txt files on the same folder than your movie and they are not subtitles but are auto-detected.

modules/demux/subtitle.c

@@ -442,7 +442,7 @@ static int Open ( vlc_object_t *p_this )
     /* Quit on unknown subtitles */
     if( p_sys->i_type == SUB_TYPE_UNKNOWN )
     {
-        msg_Err( p_demux, "failed to recognize subtitle type" );
+        msg_Warn( p_demux, "failed to recognize subtitle type" );
         free( p_sys );
         return VLC_EGENERIC;
     }