refactor(tvix/eval): implement error variant for path resolution

There are multiple things that can theoretically fail while resolving
a path, as some of it includes I/O. A new error variant has been added
for this and appropriate errors have been introduced.

Change-Id: Ie222245425207dabbf203166eb5ed1eec0114483
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6184
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/errors.rs

@@ -23,6 +23,9 @@ pub enum Error {
         lhs: &'static str,
         rhs: &'static str,
     },
+
+    // Resolving a user-supplied path literal failed in some way.
+    PathResolution(String),
 }
 
 impl Display for Error {