fix(tvix/errors): display a useful intermediate error representation

Change-Id: If979a2aa21444320427f54e6530a55cab873856b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6099
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/errors.rs

@@ -10,7 +10,7 @@ pub enum Error {
 
 impl Display for Error {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        writeln!(f, "error")
+        writeln!(f, "{:?}", self)
     }
 }