override toString of Exception

[David]

 > This behavior makes sense to me because printing the backtrace should
 > concern the application, not the exception itself. If the application
 > does not want the backtrace printed, it can handle all of the exceptions.
 >
 > Ali

I just want to add color to my exceptions and the easiest way is to 
override toString, that is not possible since, the printing-code in 
druntime doesn't call exception.toString, but reimplements it's default 
behaviour.

I "fixed" it:
https://github.com/D-Programming-Language/druntime/pull/331