Docstring for rbac_rule_validation _get_exception_type

Add a detailed docstring for _get_exception_type and refactored it to:
  - ensure that integer is passed in via six.integer_types
  - add allowed error codes to global variable `_SUPPORTED_ERROR_CODES`
    to make it clear that no others should currently be permitted

Change-Id: Ib0346f4edb0aa4b86b3c0baa2246f7fbd3f4cd24
1 file changed