feat(tvix/eval): implement 'throw' and 'abort' builtins

These do essentially the same, but return different error variants as
upstream Nix considers `throw` to be (sometimes) catchable.

Change-Id: I1a9ea84567d46fb37287dbf3f3f67052f9382cca
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6259
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/errors.rs

@@ -38,6 +38,10 @@ pub enum ErrorKind {
     ParseErrors(Vec<rnix::parser::ParseError>),
 
     AssertionFailed,
+
+    // These are user-generated errors through builtins.
+    Throw(String),
+    Abort(String),
 }
 
 #[derive(Clone, Debug)]