Fix user not found state status

[bobbyiliev]

Fixes #637

│ Error: HTTP request error: status 404, response: 
│ 
│   with materialize_user.example_user,
│   on main.tf in resource "materialize_user" "example_user":
│  167: resource "materialize_user" "example_user" {

[arusahni]

My feedback is more of a nice-to-have than something that should be a blocker in getting this bugfix out.

[arusahni]

Digging into the Frontegg helpers, it seems as if we juggling this a few ways:

1. ReadUser calls doRequest, which returns nil, fmt.Errorf(...) for 404 errors.
2. Since err != nil, we immediately return that same result from ReadUser
3. Further along in ReadUser (after we've early-returned out), we invoke an API error handler for non-200 status codes and return a string with different formatting from the Frontegg helper in step 1.

Ideally we can consolidate this string formatting to be consistent across endpoints, or even include a status code (by using an Error struct).

[bobbyiliev]

Good catch @arusahni! Thank you for this!

I've created a tracking issue for this: #641