Gitlab returns a 404 for users which are not members of a project which
is not distinguishable from the project not existing or any other error.

Fixes #2.

It isn't necessary (yet).

Lifetime issues.

It still returns an array, but only one or zero can be returned.

Got bored typing up docs part of the way through. Some of the more
obscure entities are left untouched.

Objects meant to be passed to POST or PUT endpoints are not represented
here (though they should be added).

Fixes #1.

Also add a workaround which has a much smaller edge case.