handle: A Range that handles an Exception

When a Exception occurs inside a range chaining the whole chain dies usually. The Handle range fixes that, as it catches a defined exception and calls an user defined handler on it.
Please avoid using selective imports. Until https://issues.dlang.org/show_bug.cgi?id=314 is fixed, they're horribly broken. And if you're just using them in your unit tests, you can import `std.conv` in the `unittest` blocks.