refactor: reduce coupling

evaluator/nixjit_error/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "nixjit_error"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+thiserror = "2.0"

evaluator/nixjit_error/src/lib.rs

@@ -0,0 +1,17 @@
+use thiserror::Error;
+
+pub type Result<T> = core::result::Result<T, Error>;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("error occurred during parse stage: {0}")]
+    ParseError(String),
+    #[error("error occurred during downgrade stage: {0}")]
+    DowngradeError(String),
+    #[error("error occurred during variable resolve stage: {0}")]
+    ResolutionError(String),
+    #[error("error occurred during evaluation stage: {0}")]
+    EvalError(String),
+    #[error("unknown error")]
+    Unknown,
+}