logging errors/problems instead of silently failing

[LucFoley]

I noticed that for a given folder/album, if a problem occurs, like for example, your tool is not able to read a music file because it is broken, nothing happens. RSGain will just drop trying to tag all the files for the album (I am using it in easy mode with default settings, so having the album and track tags). It would be nice if such occurences were to be logged, so that the user is made aware of problems.