This has several goals:

* Remove `LambdaType.UNKNOWN`. Functions with this non-type are useless
  and should not be presented to the user.

* Don't return 404 from the list endpoint if one function cannot be loaded.
  This prevents one bad function from essentially disabling the entire
  serverless function feature. Instead, log the error and ignore the
  function.

* Don't return 404 from other endpoints either when the problem is a bad
  function. This is not a client problem. Raise an exception and let Django
  log it and return a 500.

* Remove HTTP codes from `LambdaFunction`, to improve separation of
  concerns.