Return 2-element tuple from execute_request, even on errors

Returning None on urllib exceptions meant callers throwing a TypeError due to
expecting a two-element tuple of the (response, code) format.

Instead of changing callers, let's just be consistent in returning the same
number of elements. Callers are still expected to check if response is None.
Merge pull request #4 from birknilson/always-return-tuple-from-execute-request

Return 2-element tuple from execute_request, even on errors