Fix so maxlag is handled correctly

- If database lag exceeds max lag, the API will still send 200, not 503.
  This was only added to documentation after bugzilla:31156.
  Ref. <https://www.mediawiki.org/wiki/Manual:Maxlag_parameter>
- In addition, Requests doesn't raise an exception when status code != 200
  unless raise_for_status() is called, so we should call that to catch
  any other 5xx errors such as Squid errors.
- Replace print statements with logging

Note that responses>=0.3.0 is needed to run the test suite now