Better failure when screenshot doesn't save

* Non-writable paths would raise JSON::ParseError
* Now a clearer error is raised

spec/driver_rendering_spec.rb

@@ -90,4 +90,14 @@ describe Capybara::Webkit::Driver, "rendering an image" do
       driver.evaluate_script('window.innerHeight').should eq 600
     end
   end
+
+  context "with invalid filepath" do
+    before do
+      @file_name = File.dirname(@file_name)
+    end
+
+    it "raises an InvalidResponseError" do
+      expect { render({}) }.to raise_error(Capybara::Webkit::InvalidResponseError)
+    end
+  end
 end

src/Render.cpp

@@ -1,6 +1,7 @@
 #include "Render.h"
 #include "WebPage.h"
 #include "WebPageManager.h"
+#include "ErrorMessage.h"
 
 Render::Render(WebPageManager *manager, QStringList &arguments, QObject *parent) : SocketCommand(manager, arguments, parent) {
 }
@@ -14,5 +15,13 @@ void Render::start() {
 
   bool result = page()->render( imagePath, size );
 
-  finish(result);
+  if (result) {
+    finish(true);
+  } else {
+    const QString failure = QString("Unable to save %1x%2 image to %3").
+      arg(width).
+      arg(height).
+      arg(imagePath);
+    finish(false, new ErrorMessage(failure));
+  }
 }