chore(tvix/eval): implement std::error::Error for tvix_eval::Error

This makes it easier to interface this error with other crates.

Change-Id: I4947ea6097608f8c0427fb94a819ef748d94ea4b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7711
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/source.rs

@@ -15,7 +15,7 @@ use codemap::{CodeMap, Span};
 
 /// Tracks all source code in a Tvix evaluation for accurate error
 /// reporting.
-#[derive(Clone)]
+#[derive(Clone, Debug)]
 pub struct SourceCode(Rc<RefCell<CodeMap>>);
 
 impl SourceCode {