fix: update outdated error messages in `fastapi_proxy_lib.fastapi`

[codecov[bot]]

Codecov Report

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

Comparison is base (0155d89) 96.71% compared to head (5844f2b) 96.71%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4 +/- ## ======================================= Coverage 96.71% 96.71% ======================================= Files 9 9 Lines 457 457 Branches 66 66 ======================================= Hits 442 442 Misses 9 9 Partials 6 6 ```

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