fn get_critical_variable(var_name: String) -> Result<String, CredentialsError> {
    non_empty_env_var(&var_name)
        .ok_or_else(|| CredentialsError::new(format!("No (or empty) {} in environment", var_name)))
}
Force an error if we do not see the particular variable name in the env.
rusoto/credential/src/environment.rs