Fix "foo: no such file or directory" test failure, and normalize crea…

…tion of custom error to always depend on if os.IsNotExist(err) so we don't hide other errors that might crop up in these tests

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

buildfile.go

@@ -288,6 +288,9 @@ func (b *buildFile) CmdVolume(args string) error {
 func (b *buildFile) checkPathForAddition(orig string) error {
 	origPath := path.Join(b.contextPath, orig)
 	if p, err := filepath.EvalSymlinks(origPath); err != nil {
+		if os.IsNotExist(err) {
+			return fmt.Errorf("%s: no such file or directory", orig)
+		}
 		return err
 	} else {
 		origPath = p
@@ -297,7 +300,10 @@ func (b *buildFile) checkPathForAddition(orig string) error {
 	}
 	_, err := os.Stat(origPath)
 	if err != nil {
-		return fmt.Errorf("%s: no such file or directory", orig)
+		if os.IsNotExist(err) {
+			return fmt.Errorf("%s: no such file or directory", orig)
+		}
+		return err
 	}
 	return nil
 }
@@ -313,7 +319,10 @@ func (b *buildFile) addContext(container *Container, orig, dest string) error {
 	}
 	fi, err := os.Stat(origPath)
 	if err != nil {
-		return fmt.Errorf("%s: no such file or directory", orig)
+		if os.IsNotExist(err) {
+			return fmt.Errorf("%s: no such file or directory", orig)
+		}
+		return err
 	}
 	if fi.IsDir() {
 		if err := archive.CopyWithTar(origPath, destPath); err != nil {