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/GithubException.py

@@ -123,3 +123,9 @@ class TwoFactorException(GithubException):
     """
     Exception raised when Github requires a onetime password for two-factor authentication
     """
+
+
+class IncompletableObject(GithubException):
+    """
+    Exception raised when we can not request an object from Github because the data returned did not include a URL
+    """