[#634] Handle lack of valid assignments

* Return 403 - Invalid assignment
* TODO: Use keywords instead of message?
* TODO: writeOkResponse implies a successful response. Consider alternatives
[#634] Avoid unnecessary casting

* Rely on RestResponse parameter, and its base properties
[#634] Merge branch 'develop' into issue/634

Conflicts:
	GAE/src/org/waterforpeople/mapping/app/web/SurveyedLocaleServlet.java
[#634] Add message upon error
Merge pull request #659 from akvo/issue/634

Issue/634