obj.rb

module Api
  class ApiError < StandardError  
    attr_reader :response_body

    def initialize(response_body)
      throw "Sorry, send response body hash" unless response_body.is_a? ::Hash
      @response_body = response_body
      
      super(to_s) if error_infos
    end

    def error_infos
      Hash.include Api::CoreExtensions::Hash::Deep
      @response_body.values_at_deep :error_info
    end

    def raise_if_needed
      raise self if error_infos.any?
    end

    def to_s
      error_infos.map do |m| 
        "[#{m[:error_code]}] #{m[:error_message]} #{m[:error_data]}"
      end.join(', ')
    end
  end
end

test.rb

require 'test_helper'

module Api
  class ApiErrorTest < ActiveSupport::TestCase
    def response_body
      {
        "body" => 
        [
          {:error_info => 
            {
              :error_code => 123,
              :error_message => "First error",
              :error_data => "payload1"
            },
          },
          {:error_info =>
            {
              :error_code => 234,
              :error_message => "Second error",
              :error_data => "payload2"
            }
          }
        ]
      }
    end

    test "stringifies nicely" do
      ex = Api::ApiError.new(response_body)
      assert_equal "[123] First error payload1, [234] Second error payload2", ex.to_s
    end

    test "raises if needed" do
      ex = Api::ApiError.new(response_body)

      assert_raises(Api::ApiError) {
        ex.raise_if_needed
      }
    end

    test "accepts only hashes" do
      ex = assert_raises(Exception) { ApiError.new("normal string message") }
      assert ex.message.include? "hash"
    end
  end
end