code: func (c *Config) GetRequired(key string) Value {
  	val, err := c.Get(key)
  	if err != nil {
  		configErrorf("Configuration parameter %s is mandatory", key)
  	}
  	return val
  }
docs: // GetRequired : get the Value associated to a key found in Config structure and exit if
  // not available
  // return (Value) : Value associated to key if it exists