Fix: Include 404 check for okta_app_user resource at App level

[ymylei]

If the top level application is deleted, all the downstream assignments go with it.

This patch includes a 404 check when the app resource is being verified in the Read stage and marks the resources as gone if it cannot find the app.

[ymylei]

Test results:

➜  terraform-provider-okta git:(app_users_404_patch) TEST_FILTER="TestAccOktaAppUser" make testacc
go mod tidy
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v  -run TestAccOktaAppUser -timeout 120m
?       github.com/okta/terraform-provider-okta [no test files]
=== RUN   TestAccOktaAppUser_crud
--- PASS: TestAccOktaAppUser_crud (43.34s)
=== RUN   TestAccOktaAppUser_retain
--- PASS: TestAccOktaAppUser_retain (21.06s)
PASS
ok      github.com/okta/terraform-provider-okta/okta    64.624s
testing: warning: no tests to run
PASS
ok      github.com/okta/terraform-provider-okta/okta/internal/apimutex  0.073s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/okta/terraform-provider-okta/okta/internal/transport 0.109s [no tests to run]
?       github.com/okta/terraform-provider-okta/sdk     [no test files]