Move fault handling related in ActorCell to ActorCell.FaultHandling.cs

This is to prepare for upcoming refactorings, in order to minimize merge conflicts.
No actual code changes, just moved functions to ActorCell.FaultHandling.cs in the same order as the appear in actor\dungeon\FaultHandling.scala
Merge pull request #474 from HCanber/faulthandling

Move fault handling related functions in ActorCell to ActorCell.FaultHandling.cs