Step definition to test for the presence of error messages (and display discrepancies)

error_steps.rb

Then /^I should see( only)? the following error messages?( in order)?:$/ do |only, ordered, table|
  # Uses Capybara's `all` CSS selector to find error messages in Rails' default
  # error messages output format
  errors = all(:css, "#errorExplanation li").map { |error| [error.text] }
  errors_expected = table.raw
  errors_actual = only ? errors : (errors & table.raw)
  unless ordered
    # Cucumber cares about the order of tables, If we don't then we need to sort
    # both sets of messages using the same criteria
    errors_actual = errors_actual.sort
    errors_expected = errors_expected.sort
  end
  # To display the differences between expected and actual tables, Cucumber
  # needs the table to have column headings (we only have one column)
  Cucumber::Ast::Table.new([["Error"]] + errors_actual).diff!([["Error"]] + errors_expected)
end

my.feature

    # Sample ways to use this step

    And I should see the following error messages:
    | Start date can't be blank          |
    | Maximum attendance is not a number |

    And I should see only the following error message:
    | Email has already been taken |

    And I should see the following error messages in order:
    | Player 1 doesn't have enough credit |
    | Player 3 has been banned            |