chore(tvix/eval): bootstrap some evaluator boilerplate

Change-Id: I7770a20948d18a8506c2418dea21202aa21a6ddc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6064
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/errors.rs

@@ -0,0 +1,12 @@
+use std::fmt::Display;
+
+#[derive(Debug)]
+pub struct Error {}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        writeln!(f, "error")
+    }
+}
+
+pub type EvalResult<T> = Result<T, Error>;