Now user can see more userful information then earlier.
In particular, GetDescription function return text with
information with "required" line.
See example below:
Environment variables:
  GREETING string
        Greeting phrase
        Default: "Hello!"
        Required: true
  DB_HOST string
        Database host
        Required: true
  DB_PORT string
        Database port
        Required: true
  DB_USER string
        Database user name
        Required: true
  DB_PASSWORD string
        Database user password
        Required: true
  DB_NAME string
        Database name
        Required: true

Error from ReadEnv contains information not only about
structure's field, but about env variables for this field as well.
"field "Host" is required but the value is not provided.
Envs names: "SRV_HOST HOST""

Also, all test changed for the last changes.