Added empty invalid_element_errors method. Driver does not inherit from Base driver and was intermittently causing the following error:

undefined method `invalid_element_errors' for #<Capybara::Driver::Webkit:0x...> (NoMethodError)
      (eval):2:in `has_content?'

lib/capybara/driver/webkit.rb

@@ -126,6 +126,10 @@ class Capybara::Driver::Webkit
     @cookie_jar ||= CookieJar.new(browser)
   end
 
+  def invalid_element_errors
+    []
+  end
+
   private
 
   def url(path)