Shows a more helpful message if there's a parsing error when reading the
index file.
Also fixes a clazy warning with using the `.data()` method in a
temporary QByteArray object.
For now this doesn't mean much, but it will help when we need data
exclusive from the metadata, such as addon id and mod provider.
Also removes the metadata when the mod is deleted, and make the Mod.h
file a little more pleasing to look at :)
