Add an IncompletableObject exception (#1227)

If a returned object from GitHub does not include an URL, that object
can not be completed, and used to fail with an obscure traceback saying
NoneType has no method startswith. Check for this, and raise
IncompletableObject instead.

github/GithubObject.py

@@ -266,6 +266,8 @@ class CompletableGithubObject(GithubObject):
             self.__complete()
 
     def __complete(self):
+        if self._url.value is None:
+            raise GithubException.IncompletableObject(400, "Returned object contains no URL")
         headers, data = self._requester.requestJsonAndCheck(
             "GET",
             self._url.value