catch ReadError when calling item.read()
write: fix undefined reference to MediaFile
revise docs for expanded RG plugin

Among other things, this clarifies which options are available with each
backend.
rename Item.media_fields to _media_fields

Following the convention of the other field sets and such. This helps avoid
any confusion with user-specified fields (although it's unlikely people will
want to name a flexible field "media_fields" :).
MediaFile.update() no longer writes files

You now call update() followed by save(). This is mainly because the
implementation no longer performs lazy updates, so the need to pair tag
setting and writing together was diminished. Benefits in cleanliness:
- No need to duplicate the id3v2 option.
- Exception handling can be more precise.
- More closely resembles dict.update().