[phobos] Drought of functions that throw exceptions

[Richard Palme]

Is there a reason why so few functions in phobos throw 
exceptions? For example there is std.bitmanip.BitArray.flip that 
gets an index as argument and flips the bit that the index 
references, and this function does not throw an exception when 
given an invalid argument.

I'd be willing to fix this, but first I wanted to ask if there 
might be a reason for this. I think that throwing an exception 
should be preferable to a segmentation fault (which happens in 
the case of BitArray.flip).