You can use these step definitions:

Then /^I should not see an error$/ do
  (200 .. 399).should include(page.status_code)
end

Then /^I should see an error$/ do
  (400 .. 599).should include(page.status_code)
end

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Note that you need to tag the scenario with @allow-rescue to test that an error is shown like this

@allow-rescue
Scenario: Accessing the admin area requires a login
  When I go to the admin area
  Then I should see an error

These step definitions will not work for @javascript scenarios using the selenium web-driver

@allow-rescue @javascript
Scenario: Accessing the admin area requires a login
  When I go to the admin area
  Then I should see an error
  # This throws `Capybara::Driver::Base#status_code (Capybara::NotSupportedByDriverError)`