Handle missing storage files (#576)

* Move `validate_file` to `oauth2client.util`
* Warn user if storage file is missing
* Raise an `IOError` exception if the given filename is a directory.
* Raise an `IOError` exception if the given filename is a symbolic link.
      (Previously raised `CredentialsFileSymbolicLinkError`)
* (test) Expanding single-letter variables
* (test) `assertEqual(None, <obj>)` -> `assertIsNone(<obj>)`
* (test) `assertNotEqual(None, <obj>)` -> `assertIsNotNone(<obj>)`