title: User defined http status code on Google App Engine
body: <p>I have the following handler that gets the content from the received request:</p>
  
  <pre><code>class DummyHandler(webapp2.RequestHandler):
      def get(self):
          try:
               msg = cgi.escape(self.request.get('content'))
               if msg is None:
                  raise IllegalContent
          except IllegalContent as ex:
              logging.error("Illegal content received")
              self.error(RequestErrorCodes.ILLEGAL_CONTENT)
          except Exception as ex:
              logging.error(ex) 
              self.error(RequestErrorCodes.UNEXPECTED_ERROR)
  </code></pre>
  
  <p>When I run the unittest, I get the following error:</p>
  
  <pre><code>    self.error(RequestErrorCodes.UNEXPECTED_ERROR)
    File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/lib/webapp2/webapp2.py", line 582, in error
      self.response.status = code
  AttributeError: can't set attribute
  </code></pre>
  
  <p>I need to define user specific error codes in the http response for my client to handle. Is this supported in Google App Engine?</p>