Add matcher for javascript error checking
user
```ruby
module Capybara
  module Webkit
    module RspecMatchers
      extend RSpec::Matchers::DSL

      matcher :have_errors do |expected|
        match do |actual|
          actual = resolve(actual)
          actual.error_messages.any?
        end

        failure_message_for_should do |actual|
          "Expected Javascript errors, but there were none."
        end
        failure_message_for_should_not do |actual|
          actual = resolve(actual)
          "Expected no Javascript errors, got:\n#{error_messages_for(actual)}"
        end

        def error_messages_for(obj)
          obj.error_messages.map do |m|
            "  - #{m[:message]}"
          end.join("\n")
        end

        def resolve(actual)
          if actual.respond_to? :page
            actual.page.driver
          elsif actual.respond_to? :driver
            actual.driver
          else
            actual
          end
        end
      end

    end
  end
end
```
assistant