Handle NUS auth errors gracefully

[jloh02]

Describe the bug

During CPEx, login to NUS auth using email instead of NUSNET ID will cause a redirect to /api/nus/auth/login without a secondary redirect back to CPEx page, causing "An unexpected error has occurred" error to appear.

Expected behavior

A proper error page showing that the user was not fully authenticated and need to take the following steps to rectify:

• Clear browser cache/Incognito
• Login again using NUSNETID