Raise an error when scope not found on page

lib/webcat/session.rb

@@ -80,6 +80,7 @@ def has_css?(path, options={})
   end
 
   def within(scope)
+    raise Webcat::ElementNotFound, "scope '#{scope}' not found on page" if find(scope).empty?
     scopes.push(scope)
     yield
     scopes.pop

spec/session_spec.rb

@@ -482,6 +482,13 @@ def extract_results(session)
           @session.body.should include('Hello world')
         end
       end
+
+      it "should raise an error if the scope is not found on the page" do
+        running {
+          @session.within("//div[@id='doesnotexist']") do
+          end
+        }.should raise_error(Webcat::ElementNotFound)
+      end
     end
 
     context "with forms" do