Less repetitive error messages in internal/cache

Many instances of errors.Wrap in this package would produce messages
like "Open: open <filename>: no such file or directory"; those now omit
the first "Open:" (or "Stat:", or "MkdirAll"). The function readVersion
now appends its own name to the error message, rather than the function
that failed, to make it easier to spot. Other function names (e.g.,
Load) are already added further up in the call chain.

internal/cache/file.go

@@ -48,13 +48,13 @@ func (c *Cache) load(h restic.Handle, length int, offset int64) (io.ReadCloser,
 
 	f, err := fs.Open(c.filename(h))
 	if err != nil {
-		return nil, errors.Wrap(err, "Open")
+		return nil, errors.WithStack(err)
 	}
 
 	fi, err := f.Stat()
 	if err != nil {
 		_ = f.Close()
-		return nil, errors.Wrap(err, "Stat")
+		return nil, errors.WithStack(err)
 	}
 
 	if fi.Size() <= crypto.Extension {
@@ -94,15 +94,11 @@ func (c *Cache) saveWriter(h restic.Handle) (io.WriteCloser, error) {
 	p := c.filename(h)
 	err := fs.MkdirAll(filepath.Dir(p), 0700)
 	if err != nil {
-		return nil, errors.Wrap(err, "MkdirAll")
+		return nil, errors.WithStack(err)
 	}
 
 	f, err := fs.OpenFile(p, os.O_CREATE|os.O_EXCL|os.O_WRONLY, 0400)
-	if err != nil {
-		return nil, errors.Wrap(err, "Create")
-	}
-
-	return f, err
+	return f, errors.WithStack(err)
 }
 
 // Save saves a file in the cache.
@@ -133,7 +129,7 @@ func (c *Cache) Save(h restic.Handle, rd io.Reader) error {
 
 	if err = f.Close(); err != nil {
 		_ = c.remove(h)
-		return errors.Wrap(err, "Close")
+		return errors.WithStack(err)
 	}
 
 	return nil