Allow fallback decoder overwrite
Allow fallback decoder overwrite

To fix the runtime crashes in difficulty calculation components, remove
the check for pre-existing fallback registration along with
the exception. The xmldoc for the registration function has been
extended to make users aware of possible consequences of calling it.

UselessToucan/osu,johnneijzen/osu,smoogipooo/osu,smoogipoo/osu,ppy/osu,EVAST9919/osu,NeoAdonis/osu,smoogipoo/osu,ppy/osu,smoogipoo/osu,peppy/osu,2yangk23/osu,peppy/osu,NeoAdonis/osu,ZLima12/osu,EVAST9919/osu,UselessToucan/osu,ppy/osu,peppy/osu,peppy/osu-new,NeoAdonis/osu,johnneijzen/osu,UselessToucan/osu,ZLima12/osu,2yangk23/osu