@nogc and exceptions

[Dicebot via Digitalmars-d]

I am obviously in favor of simply calling recurrent mutable 
exception chains illegal (probably even immutable ones until we 
fix const correctness of druntime exception handling).

Reason is simple : it doesn't require any changes in existing 
code and is exactly the way we already do it in Sociomantic :)

To make it reliable Exception chain may need to be updated to 
doubly-linked list to be able to efficiently verify that new 
exception is not already present in the chain. I don't see any 
costly overhead implications from that though.