Null references redux

[Sean Kelly]

== Quote from Sean Kelly (sean at invisibleduck.org)'s article
>
> One thing I'm not entirely sure about is whether the signal handler will always
> have a valid, C-style call stack tracing back into user code.  These errors are
> triggered by hardware, and I really don't know what kind of tricks are common
> at that level of OS code.  longjmp() doesn't have this problem because it doesn't
> care about the call stack--it just swaps some registers and executes a JMP.  I
> don't suppose anyone here knows more about the feasibility of throwing
> exceptions from signal handlers at all?  I'll ask around some OS groups and
> see what people say.

I was right, it is illegal to throw an exception from a signal handler.  And worse,
it's illegal to call malloc from a signal handler, so you can't safely create an
exception object anyway.  Heck, I'm not sure it's even safe to perform IO from
a signal handler, so tracing directly from within the handler won't even work
reliably.  In short, while I'm totally fine with people using this in their own
code, it's too unreliable to make an "official" solution by adding it to Druntime.