Add handler for failed CPEX logins

[jloh02]

Context

Receive a "Unexpected error has occurred" message when trying to login to CPEX page

Implementation

Add extra handler. defaultRescue should not occur instead?

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Name	Status	Preview	Comments	Updated (UTC)
nusmods-export	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 15, 2024 8:22am

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 54.87%. Comparing base (2d4743d) to head (c230030). Report is 37 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3835 +/- ## ========================================== + Coverage 53.87% 54.87% +1.00% ========================================== Files 274 274 Lines 6032 6066 +34 Branches 1449 1453 +4 ========================================== + Hits 3250 3329 +79 + Misses 2782 2737 -45 ```

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