Fix KeyError in flask.py

When submitting the grade form in the sample flask app
(at /grade), pylti gave me this error:

	KeyError: 'lis_outcome_service_url'

This commit turns the KeyError into a documented exception. I'm
not too familiar with LTI yet so I'm not sure if this behavior
is desired, but seems better to me.

pylti/flask.py

@@ -208,7 +208,10 @@ def response_url(self):
         uses PYLTI_URL_FIX map to support edX dev-stack
 
         :return: remapped lis_outcome_service_url
+        :exception: LTIException if lis_outcome_service_url isn't present.
         """
+        if not 'lis_outcome_service_url' in session:
+            raise LTIException('lis_outcome_service_url isn\'t present.')
         url = session['lis_outcome_service_url']
         app_config = self.lti_kwargs['app'].config
         urls = app_config.get('PYLTI_URL_FIX', dict())