changed behavior for missing user, in the `SetCurrentUser` middleware

[sio4]

What is being done in this PR?

In the scaffolded actions/users.go, middleware SetCurrentUser just returned a general error which causes a 404 error. However, it could be better if the middleware generates Unauthorized or just redirects users to the login page. With this change, the middleware redirects the user to the login page.

fixes #30

see also: https://github.com/gobuffalo/buffalo-auth/issues/30#issuecomment-1416676908

What are the main choices made to get to this solution?

• the selected solution is redirecting the user to the login page. it could be nice for interactive applications.
• however, since this is just a scaffold code, users can modify their application with their own use cases.

List the manual test cases you've covered before sending this PR:

• tested with a simple todo application