fix(overlay): remove potential panic (fixes #23)

src/impls/overlay.rs

@@ -3,6 +3,7 @@
 use crate::{FileSystem, SeekAndRead, VfsError, VfsMetadata, VfsPath, VfsResult};
 use std::collections::HashSet;
 use std::io::Write;
+use crate::error::VfsResultExt;
 
 /// An overlay file system combining several filesystems into one, an upper layer with read/write access and lower layers with only read access
 ///
@@ -152,7 +153,7 @@ impl FileSystem for OverlayFS {
     }
 
     fn exists(&self, path: &str) -> VfsResult<bool> {
-        if self.whiteout_path(path).expect("whiteout_path").exists()? {
+        if self.whiteout_path(path).with_context(|| "whiteout_path")?.exists()? {
             return Ok(false);
         }
         self.read_path(path)