RFC: make expected 404's easier to handle

[AEnterprise]

On many endpoints getting a 404 is normal and expected (getting a member, user, invite, ...). There are many cases where these will give you a 404 without anything being broken.

This can simply be because the user isn't on the server (anymore), the invite code provided to you by the user isn't valid, ....

Right now if you want to handle these gracefully you need to start wrapping each one of these calls and do several layers of error info parsing to figure out if this was a 404 or a generic failure. So i'm proposing the following 2 changes to make these much more sane to deal with for lib consumers:

1. Have a "NotFound" error in the top level of the error enum to more easily match on these vs other generic http errors
2. change these functions where it is expected to get regular 404's to Optional (like get member, get invite, ...)