tar: changes in error handling

- Adopt %w as Go 1.12 is no longer supported
- Fix check for permission to open a file: if something that was not a
permission error happened, the code could panic.

tar.go

@@ -47,7 +47,7 @@ func createTarStream(srcPath, dockerfilePath string) (io.ReadCloser, error) {
 		}
 		keepThem, err := fileutils.Matches(includeFile, excludes)
 		if err != nil {
-			return nil, fmt.Errorf("cannot match .dockerfile: '%s', error: %s", includeFile, err)
+			return nil, fmt.Errorf("cannot match .dockerfileignore: '%s', error: %w", includeFile, err)
 		}
 		if keepThem {
 			includes = append(includes, includeFile)
@@ -85,7 +85,7 @@ func validateContextDirectory(srcPath string, excludes []string) error {
 
 		if err != nil {
 			if os.IsPermission(err) {
-				return fmt.Errorf("can't stat '%s'", filePath)
+				return fmt.Errorf("cannot stat %q: %w", filePath, err)
 			}
 			if os.IsNotExist(err) {
 				return nil
@@ -101,8 +101,8 @@ func validateContextDirectory(srcPath string, excludes []string) error {
 
 		if !f.IsDir() {
 			currentFile, err := os.Open(filePath)
-			if err != nil && os.IsPermission(err) {
-				return fmt.Errorf("no permission to read from '%s'", filePath)
+			if err != nil {
+				return fmt.Errorf("cannot open %q for reading: %w", filePath, err)
 			}
 			currentFile.Close()
 		}
@@ -114,7 +114,7 @@ func parseDockerignore(root string) ([]string, error) {
 	var excludes []string
 	ignore, err := ioutil.ReadFile(path.Join(root, ".dockerignore"))
 	if err != nil && !os.IsNotExist(err) {
-		return excludes, fmt.Errorf("error reading .dockerignore: '%s'", err)
+		return excludes, fmt.Errorf("error reading .dockerignore: %w", err)
 	}
 	excludes = strings.Split(string(ignore), "\n")