Mechanism for catching errors for tests

[davidzchen]

From our discussion on Tuesday: in general, we should not add a catch mechanism to the language, but having a specific mechanism such as assertThrows or assertError would be useful for testing.

[copumpkin]

This would be great. There's a parallel discussion going on in Nix right now about whether their equivalent to this (called tryEval) should return details about the error. The argument against providing details is that they can reveal evaluation order, which is pretty icky in a pure language like this.

[copumpkin]

See NixOS/nix#1000 (comment).

[fkz]

I also need something like this for NixOS/nixos-homepage#78 where I need to be able to catch all errors so that evaluation of one package can't break evaluation of an other one. (see here: fkz/nix@61dc5a7)