Fix Issue 10420.

Mark `core.exception` functions with `pure`, `nothrow`, and friends.
Require user-supplied assert hander to be `nothrow`.

Issue URL: http://d.puremagic.com/issues/show_bug.cgi?id=10420
Make assert handler in line with the other ones.
Merge pull request #527 from denis-sh/add-`core.exception`-functions-attributes

Fix Issue 10420 - Incorrect function attributes in `core.exception`