stat: recompose and propagate error instead of just logging it

Fixes #801.

Recompose and propagate encountered errors along with their
status/run-level codes, instead of just logging them.

* Exhibit:
```shell
$ drive stat goos-goarch && echo "success"; echo $?
stat: /goos-goarch err: remote path doesn't exist
9
```

src/misc.go

@@ -806,6 +806,19 @@ func combineErrors(prevErr, supplementaryErr error) error {
 	return newErr
 }
 
+// copyErrStatus copies the error status code from fromErr
+// to toErr only if toErr and fromErr are both non-nil.
+func copyErrStatusCode(toErr, fromErr error) error {
+	if toErr == nil || fromErr == nil {
+		return toErr
+	}
+	codedErr, hasCode := fromErr.(*Error)
+	if hasCode {
+		toErr = makeError(toErr, ErrorStatus(codedErr.Code()))
+	}
+	return toErr
+}
+
 func reComposeError(prevErr error, messages ...string) error {
 	if len(messages) < 1 {
 		return prevErr

src/stat.go

@@ -37,33 +37,34 @@ func (g *Commands) Stat() error {
 }
 
 func (g *Commands) statfn(fname string, fn func(string) (*File, error)) error {
+	var err error
 	for _, src := range g.opts.Sources {
-		f, err := fn(src)
-		if err != nil {
-			g.log.LogErrf("%s: %s err: %v\n", fname, src, err)
+		f, fErr := fn(src)
+		if fErr != nil {
+			msg := fmt.Sprintf("%s: %s err: %v\n", fname, src, fErr)
+			err = reComposeError(err, msg)
+			err = copyErrStatusCode(err, fErr)
 			continue
 		}
 
 		if g.opts.Md5sum {
-
 			src = f.Name // forces filename if -id is used
 
 			// md5sum with no arguments should do md5sum *
 			if f.IsDir && rootLike(g.opts.Path) {
 				src = ""
 			}
-
 		}
 
-		err = g.stat(src, f, g.opts.Depth)
-
-		if err != nil {
-			g.log.LogErrf("%s: %s err: %v\n", fname, src, err)
+		if fErr = g.stat(src, f, g.opts.Depth); fErr != nil {
+			msg := fmt.Sprintf("%s: %s err: %v\n", fname, src, fErr)
+			err = reComposeError(err, msg)
+			err = copyErrStatusCode(err, fErr)
 			continue
 		}
 	}
 
-	return nil
+	return err
 }
 
 func prettyPermission(logf log.Loggerf, perm *drive.Permission) {