Fix stat/lstat error checking in a few places.

closes #6929. Please review.
Here’s the adjusted function:

```
func ValidateContextDirectory(srcPath string) error {
	var finalError error

	filepath.Walk(filepath.Join(srcPath, "."), func(filePath string, f os.FileInfo, err error) error {
		// skip this directory/file if it's not in the path, it won't get added to the context
		_, err = filepath.Rel(srcPath, filePath)
		if err != nil && os.IsPermission(err) {
			return nil
		}

		// skip checking if symlinks point to non-existing files, such symlinks can be useful
		lstat, err := os.Lstat(filePath)
		if err != nil {
			return err
		}
		if lstat.Mode()&os.ModeSymlink == os.ModeSymlink {
			return fmt.Errorf("ValidateContextDirectory: %s is a symlink", filePath)
		}

		if !f.IsDir() {
			currentFile, err := os.Open(filePath)
			if err != nil && os.IsPermission(err) {
				finalError = fmt.Errorf("no permission to read from '%s'", filePath)
				return err
			} else {
				currentFile.Close()
			}
		}
		return nil
	})
	return finalError
}
```
> On Jul 12, 2014, at 6:53 AM, Tibor Vass <notifications@github.com> wrote:
> 
> In utils/utils.go:
> 
> > @@ -889,8 +889,8 @@ func ValidateContextDirectory(srcPath string) error {
> >  			return err
> >  		}
> >  		// skip checking if symlinks point to non-existing files, such symlinks can be useful
> > -		lstat, _ := os.Lstat(filePath)
> > -		if lstat.Mode()&os.ModeSymlink == os.ModeSymlink {
> > +		lstat, err := os.Lstat(filePath)
> Did you address the return err below?
> 
> —
> Reply to this email directly or view it on GitHub.
>